Support multiple async callbacks

api/include/opentelemetry/metrics/async_instruments.h

@@ -9,20 +9,22 @@
 OPENTELEMETRY_BEGIN_NAMESPACE
 namespace metrics
 {
-class AsynchronousInstrument
-{};
 
-template <class T>
-class ObservableCounter : public AsynchronousInstrument
-{};
+using ObservableCallbackPtr = void (*)(ObserverResult, void *);
 
-template <class T>
-class ObservableGauge : public AsynchronousInstrument
-{};
+class ObservableInstrument
+{
+public:
+  /**
+   * Sets up a function that will be called whenever a metric collection is initiated.
+   */
+  virtual void AddCallback(ObservableCallbackPtr, void *state) noexcept = 0;
 
-template <class T>
-class ObservableUpDownCounter : public AsynchronousInstrument
-{};
+  /**
+   * Remove a function that was configured to be called whenever a metric collection is initiated.
+   */
+  virtual void RemoveCallback(ObservableCallbackPtr, void *state) noexcept = 0;
+};
 
 }  // namespace metrics
 OPENTELEMETRY_END_NAMESPACE

api/include/opentelemetry/metrics/meter.h

@@ -51,22 +51,18 @@ class Meter
    * shared_ptr to that Observable Counter
    *
    * @param name the name of the new Observable Counter.
-   * @param callback the function to be observed by the instrument.
    * @param description a brief description of what the Observable Counter is used for.
    * @param unit the unit of metric values following https://unitsofmeasure.org/ucum.html.
-   * @param state to be passed back to callback
    */
-  virtual void CreateLongObservableCounter(nostd::string_view name,
-                                           void (*callback)(ObserverResult<long> &, void *),
-                                           nostd::string_view description = "",
-                                           nostd::string_view unit        = "",
-                                           void *state                    = nullptr) noexcept = 0;
-
-  virtual void CreateDoubleObservableCounter(nostd::string_view name,
-                                             void (*callback)(ObserverResult<double> &, void *),
-                                             nostd::string_view description = "",
-                                             nostd::string_view unit        = "",
-                                             void *state                    = nullptr) noexcept = 0;
+  virtual nostd::shared_ptr<ObservableInstrument> CreateLongObservableCounter(
+      nostd::string_view name,
+      nostd::string_view description = "",
+      nostd::string_view unit        = "") noexcept = 0;
+
+  virtual nostd::shared_ptr<ObservableInstrument> CreateDoubleObservableCounter(
+      nostd::string_view name,
+      nostd::string_view description = "",
+      nostd::string_view unit        = "") noexcept = 0;
 
   /**
    * Creates a Histogram with the passed characteristics and returns a shared_ptr to that Histogram.
@@ -91,22 +87,18 @@ class Meter
    * shared_ptr to that Observable Counter
    *
    * @param name the name of the new Observable Gauge.
-   * @param callback the function to be observed by the instrument.
    * @param description a brief description of what the Observable Gauge is used for.
    * @param unit the unit of metric values following https://unitsofmeasure.org/ucum.html.
-   * @param state to be passed back to callback
    */
-  virtual void CreateLongObservableGauge(nostd::string_view name,
-                                         void (*callback)(ObserverResult<long> &, void *),
-                                         nostd::string_view description = "",
-                                         nostd::string_view unit        = "",
-                                         void *state                    = nullptr) noexcept = 0;
-
-  virtual void CreateDoubleObservableGauge(nostd::string_view name,
-                                           void (*callback)(ObserverResult<double> &, void *),
-                                           nostd::string_view description = "",
-                                           nostd::string_view unit        = "",
-                                           void *state                    = nullptr) noexcept = 0;
+  virtual nostd::shared_ptr<ObservableInstrument> CreateLongObservableGauge(
+      nostd::string_view name,
+      nostd::string_view description = "",
+      nostd::string_view unit        = "") noexcept = 0;
+
+  virtual nostd::shared_ptr<ObservableInstrument> CreateDoubleObservableGauge(
+      nostd::string_view name,
+      nostd::string_view description = "",
+      nostd::string_view unit        = "") noexcept = 0;
 
   /**
    * Creates an UpDownCounter with the passed characteristics and returns a shared_ptr to that
@@ -132,23 +124,18 @@ class Meter
    * a shared_ptr to that Observable UpDownCounter
    *
    * @param name the name of the new Observable UpDownCounter.
-   * @param callback the function to be observed by the instrument.
    * @param description a brief description of what the Observable UpDownCounter is used for.
    * @param unit the unit of metric values following https://unitsofmeasure.org/ucum.html.
-   * @param state to be passed back to callback
    */
-  virtual void CreateLongObservableUpDownCounter(nostd::string_view name,
-                                                 void (*callback)(ObserverResult<long> &, void *),
-                                                 nostd::string_view description = "",
-                                                 nostd::string_view unit        = "",
-                                                 void *state = nullptr) noexcept = 0;
-
-  virtual void CreateDoubleObservableUpDownCounter(nostd::string_view name,
-                                                   void (*callback)(ObserverResult<double> &,
-                                                                    void *),
-                                                   nostd::string_view description = "",
-                                                   nostd::string_view unit        = "",
-                                                   void *state = nullptr) noexcept = 0;
+  virtual nostd::shared_ptr<ObservableInstrument> CreateLongObservableUpDownCounter(
+      nostd::string_view name,
+      nostd::string_view description = "",
+      nostd::string_view unit        = "") noexcept = 0;
+
+  virtual nostd::shared_ptr<ObservableInstrument> CreateDoubleObservableUpDownCounter(
+      nostd::string_view name,
+      nostd::string_view description = "",
+      nostd::string_view unit        = "") noexcept = 0;
 };
 }  // namespace metrics
 OPENTELEMETRY_END_NAMESPACE

api/include/opentelemetry/metrics/noop.h

@@ -64,56 +64,16 @@ class NoopUpDownCounter : public UpDownCounter<T>
   {}
 };
 
-template <class T>
-class NoopObservableCounter : public ObservableCounter<T>
-{
-public:
-  NoopObservableCounter(nostd::string_view name,
-                        void (*callback)(ObserverResult<T> &),
-                        nostd::string_view description,
-                        nostd::string_view unit) noexcept
-  {}
-
-  NoopObservableCounter(nostd::string_view name,
-                        void (*callback)(ObserverResult<T> &, const common::KeyValueIterable &),
-                        nostd::string_view description,
-                        nostd::string_view unit) noexcept
-  {}
-};
-
-template <class T>
-class NoopObservableGauge : public ObservableGauge<T>
+class NoopObservableInstrument : public ObservableInstrument
 {
 public:
-  NoopObservableGauge(nostd::string_view name,
-                      void (*callback)(ObserverResult<T> &),
-                      nostd::string_view description,
-                      nostd::string_view unit) noexcept
+  NoopObservableInstrument(nostd::string_view name,
+                           nostd::string_view description,
+                           nostd::string_view unit) noexcept
   {}
 
-  NoopObservableGauge(nostd::string_view name,
-                      void (*callback)(ObserverResult<T> &, const common::KeyValueIterable &),
-                      nostd::string_view description,
-                      nostd::string_view unit) noexcept
-  {}
-};
-
-template <class T>
-class NoopObservableUpDownCounter : public ObservableUpDownCounter<T>
-{
-public:
-  NoopObservableUpDownCounter(nostd::string_view name,
-                              void (*callback)(ObserverResult<T> &),
-                              nostd::string_view description,
-                              nostd::string_view unit) noexcept
-  {}
-
-  NoopObservableUpDownCounter(nostd::string_view name,
-                              void (*callback)(ObserverResult<T> &,
-                                               const common::KeyValueIterable &),
-                              nostd::string_view description,
-                              nostd::string_view unit) noexcept
-  {}
+  void AddCallback(ObservableCallbackPtr, void *state) noexcept override {}
+  void RemoveCallback(ObservableCallbackPtr, void *state) noexcept override {}
 };
 
 /**
@@ -137,19 +97,23 @@ class NoopMeter final : public Meter
     return nostd::shared_ptr<Counter<double>>{new NoopCounter<double>(name, description, unit)};
   }
 
-  void CreateLongObservableCounter(nostd::string_view name,
-                                   void (*callback)(ObserverResult<long> &, void *),
-                                   nostd::string_view description = "",
-                                   nostd::string_view unit        = "",
-                                   void *state                    = nullptr) noexcept override
-  {}
+  nostd::shared_ptr<ObservableInstrument> CreateLongObservableCounter(
+      nostd::string_view name,
+      nostd::string_view description = "",
+      nostd::string_view unit        = "") noexcept override
+  {
+    return nostd::shared_ptr<ObservableInstrument>(
+        new NoopObservableInstrument(name, description, unit));
+  }
 
-  void CreateDoubleObservableCounter(nostd::string_view name,
-                                     void (*callback)(ObserverResult<double> &, void *),
-                                     nostd::string_view description = "",
-                                     nostd::string_view unit        = "",
-                                     void *state                    = nullptr) noexcept override
-  {}
+  nostd::shared_ptr<ObservableInstrument> CreateDoubleObservableCounter(
+      nostd::string_view name,
+      nostd::string_view description = "",
+      nostd::string_view unit        = "") noexcept override
+  {
+    return nostd::shared_ptr<ObservableInstrument>(
+        new NoopObservableInstrument(name, description, unit));
+  }
 
   nostd::shared_ptr<Histogram<long>> CreateLongHistogram(
       nostd::string_view name,
@@ -167,19 +131,23 @@ class NoopMeter final : public Meter
     return nostd::shared_ptr<Histogram<double>>{new NoopHistogram<double>(name, description, unit)};
   }
 
-  void CreateLongObservableGauge(nostd::string_view name,
-                                 void (*callback)(ObserverResult<long> &, void *),
-                                 nostd::string_view description = "",
-                                 nostd::string_view unit        = "",
-                                 void *state                    = nullptr) noexcept override
-  {}
+  nostd::shared_ptr<ObservableInstrument> CreateLongObservableGauge(
+      nostd::string_view name,
+      nostd::string_view description = "",
+      nostd::string_view unit        = "") noexcept override
+  {
+    return nostd::shared_ptr<ObservableInstrument>(
+        new NoopObservableInstrument(name, description, unit));
+  }
 
-  void CreateDoubleObservableGauge(nostd::string_view name,
-                                   void (*callback)(ObserverResult<double> &, void *),
-                                   nostd::string_view description = "",
-                                   nostd::string_view unit        = "",
-                                   void *state                    = nullptr) noexcept override
-  {}
+  nostd::shared_ptr<ObservableInstrument> CreateDoubleObservableGauge(
+      nostd::string_view name,
+      nostd::string_view description = "",
+      nostd::string_view unit        = "") noexcept override
+  {
+    return nostd::shared_ptr<ObservableInstrument>(
+        new NoopObservableInstrument(name, description, unit));
+  }
 
   nostd::shared_ptr<UpDownCounter<long>> CreateLongUpDownCounter(
       nostd::string_view name,
@@ -199,19 +167,23 @@ class NoopMeter final : public Meter
         new NoopUpDownCounter<double>(name, description, unit)};
   }
 
-  void CreateLongObservableUpDownCounter(nostd::string_view name,
-                                         void (*callback)(ObserverResult<long> &, void *),
-                                         nostd::string_view description = "",
-                                         nostd::string_view unit        = "",
-                                         void *state                    = nullptr) noexcept override
-  {}
+  nostd::shared_ptr<ObservableInstrument> CreateLongObservableUpDownCounter(
+      nostd::string_view name,
+      nostd::string_view description = "",
+      nostd::string_view unit        = "") noexcept override
+  {
+    return nostd::shared_ptr<ObservableInstrument>(
+        new NoopObservableInstrument(name, description, unit));
+  }
 
-  void CreateDoubleObservableUpDownCounter(nostd::string_view name,
-                                           void (*callback)(ObserverResult<double> &, void *),
-                                           nostd::string_view description = "",
-                                           nostd::string_view unit        = "",
-                                           void *state = nullptr) noexcept override
-  {}
+  nostd::shared_ptr<ObservableInstrument> CreateDoubleObservableUpDownCounter(
+      nostd::string_view name,
+      nostd::string_view description = "",
+      nostd::string_view unit        = "") noexcept override
+  {
+    return nostd::shared_ptr<ObservableInstrument>(
+        new NoopObservableInstrument(name, description, unit));
+  }
 };
 
 /**

api/include/opentelemetry/metrics/observer_result.h

@@ -6,6 +6,7 @@
 
 #  include "opentelemetry/common/attribute_value.h"
 #  include "opentelemetry/common/key_value_iterable_view.h"
+#  include "opentelemetry/nostd/shared_ptr.h"
 #  include "opentelemetry/nostd/span.h"
 #  include "opentelemetry/nostd/string_view.h"
 #  include "opentelemetry/nostd/type_traits.h"
@@ -15,12 +16,11 @@ namespace metrics
 {
 
 /**
- * ObserverResult class is necessary for the callback recording asynchronous
+ * ObserverResultT class is necessary for the callback recording asynchronous
  * instrument use.
  */
-
 template <class T>
-class ObserverResult
+class ObserverResultT
 {
 
 public:
@@ -44,6 +44,9 @@ class ObserverResult
   }
 };
 
+using ObserverResult = nostd::variant<nostd::shared_ptr<ObserverResultT<long>>,
+                                      nostd::shared_ptr<ObserverResultT<double>>>;
+
 }  // namespace metrics
 OPENTELEMETRY_END_NAMESPACE
 #endif

examples/common/metrics_foo_library/foo_library.cc

@@ -32,12 +32,18 @@ std::map<std::string, std::string> get_random_attr()
 class MeasurementFetcher
 {
 public:
-  static void Fetcher(opentelemetry::metrics::ObserverResult<double> &observer_result, void *state)
+  static void Fetcher(opentelemetry::metrics::ObserverResult observer_result, void *state)
   {
-    double val                                = (rand() % 700) + 1.1;
     std::map<std::string, std::string> labels = get_random_attr();
     auto labelkv = opentelemetry::common::KeyValueIterableView<decltype(labels)>{labels};
-    observer_result.Observe(val /*, labelkv*/);
+    if (nostd::holds_alternative<
+            nostd::shared_ptr<opentelemetry::metrics::ObserverResultT<double>>>(observer_result))
+    {
+      double val = (rand() % 700) + 1.1;
+      nostd::get<nostd::shared_ptr<opentelemetry::metrics::ObserverResultT<double>>>(
+          observer_result)
+          ->Observe(val /*, labelkv */);
+    }
   }
 };
 }  // namespace
@@ -62,7 +68,8 @@ void foo_library::observable_counter_example(const std::string &name)
   std::string counter_name                    = name + "_observable_counter";
   auto provider                               = metrics_api::Provider::GetMeterProvider();
   nostd::shared_ptr<metrics_api::Meter> meter = provider->GetMeter(name, "1.2.0");
-  meter->CreateDoubleObservableCounter(counter_name, MeasurementFetcher::Fetcher);
+  auto counter                                = meter->CreateDoubleObservableCounter(counter_name);
+  counter->AddCallback(MeasurementFetcher::Fetcher, nullptr);
   while (true)
   {
     std::this_thread::sleep_for(std::chrono::milliseconds(500));