Restructure RegisterCallback method

CHANGELOG.md

@@ -100,6 +100,8 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - The exporter from `go.opentelemetry.io/otel/exporters/zipkin` is updated to use the `v1.16.0` version of semantic conventions.
   This means it no longer uses the removed `net.peer.ip` or `http.host` attributes to determine the remote endpoint.
   Instead it uses the `net.sock.peer` attributes. (#3581)
+- The parameters for the `RegisterCallback` method of the `Meter` from `go.opentelemetry.io/otel/metric` are changed.
+  The slice of `instrument.Asynchronous` parameter is now passed as a variadic argument. (#3587)
 
 ### Deprecated
 

example/prometheus/main.go

@@ -68,11 +68,11 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
-	_, err = meter.RegisterCallback([]instrument.Asynchronous{gauge}, func(ctx context.Context) error {
+	_, err = meter.RegisterCallback(func(ctx context.Context) error {
 		n := -10. + rand.Float64()*(90.) // [-10, 100)
 		gauge.Observe(ctx, n, attrs...)
 		return nil
-	})
+	}, gauge)
 	if err != nil {
 		log.Fatal(err)
 	}

metric/example_test.go

@@ -89,10 +89,7 @@ func ExampleMeter_asynchronous_multiple() {
 	gcCount, _ := meter.Int64ObservableCounter("gcCount")
 	gcPause, _ := meter.Float64Histogram("gcPause")
 
-	_, err := meter.RegisterCallback([]instrument.Asynchronous{
-		heapAlloc,
-		gcCount,
-	},
+	_, err := meter.RegisterCallback(
 		func(ctx context.Context) error {
 			memStats := &runtime.MemStats{}
 			// This call does work
@@ -105,6 +102,8 @@ func ExampleMeter_asynchronous_multiple() {
 			computeGCPauses(ctx, gcPause, memStats.PauseNs[:])
 			return nil
 		},
+		heapAlloc,
+		gcCount,
 	)
 
 	if err != nil {

metric/internal/global/meter.go

@@ -278,10 +278,10 @@ func (m *meter) Float64ObservableGauge(name string, options ...instrument.Float6
 //
 // It is only valid to call Observe within the scope of the passed function,
 // and only on the instruments that were registered with this call.
-func (m *meter) RegisterCallback(insts []instrument.Asynchronous, f metric.Callback) (metric.Registration, error) {
+func (m *meter) RegisterCallback(f metric.Callback, insts ...instrument.Asynchronous) (metric.Registration, error) {
 	if del, ok := m.delegate.Load().(metric.Meter); ok {
 		insts = unwrapInstruments(insts)
-		return del.RegisterCallback(insts, f)
+		return del.RegisterCallback(f, insts...)
 	}
 
 	m.mtx.Lock()
@@ -335,7 +335,7 @@ func (c *registration) setDelegate(m metric.Meter) {
 		return
 	}
 
-	reg, err := m.RegisterCallback(insts, c.function)
+	reg, err := m.RegisterCallback(c.function, insts...)
 	if err != nil {
 		otel.Handle(err)
 	}

metric/internal/global/meter_test.go

@@ -66,7 +66,7 @@ func TestMeterRace(t *testing.T) {
 			_, _ = mtr.Int64Counter(name)
 			_, _ = mtr.Int64UpDownCounter(name)
 			_, _ = mtr.Int64Histogram(name)
-			_, _ = mtr.RegisterCallback(nil, func(ctx context.Context) error { return nil })
+			_, _ = mtr.RegisterCallback(func(ctx context.Context) error { return nil })
 			if !once {
 				wg.Done()
 				once = true
@@ -86,7 +86,7 @@ func TestMeterRace(t *testing.T) {
 
 func TestUnregisterRace(t *testing.T) {
 	mtr := &meter{}
-	reg, err := mtr.RegisterCallback(nil, func(ctx context.Context) error { return nil })
+	reg, err := mtr.RegisterCallback(func(ctx context.Context) error { return nil })
 	require.NoError(t, err)
 
 	wg := &sync.WaitGroup{}
@@ -128,10 +128,10 @@ func testSetupAllInstrumentTypes(t *testing.T, m metric.Meter) (instrument.Float
 	_, err = m.Int64ObservableGauge("test_Async_Gauge")
 	assert.NoError(t, err)
 
-	_, err = m.RegisterCallback([]instrument.Asynchronous{afcounter}, func(ctx context.Context) error {
+	_, err = m.RegisterCallback(func(ctx context.Context) error {
 		afcounter.Observe(ctx, 3)
 		return nil
-	})
+	}, afcounter)
 	require.NoError(t, err)
 
 	sfcounter, err := m.Float64Counter("test_Async_Counter")
@@ -323,21 +323,21 @@ func TestRegistrationDelegation(t *testing.T) {
 	require.NoError(t, err)
 
 	var called0 bool
-	reg0, err := m.RegisterCallback([]instrument.Asynchronous{actr}, func(context.Context) error {
+	reg0, err := m.RegisterCallback(func(context.Context) error {
 		called0 = true
 		return nil
-	})
+	}, actr)
 	require.NoError(t, err)
 	require.Equal(t, 1, mImpl.registry.Len(), "callback not registered")
 	// This means reg0 should not be delegated.
 	assert.NoError(t, reg0.Unregister())
 	assert.Equal(t, 0, mImpl.registry.Len(), "callback not unregistered")
 
 	var called1 bool
-	reg1, err := m.RegisterCallback([]instrument.Asynchronous{actr}, func(context.Context) error {
+	reg1, err := m.RegisterCallback(func(context.Context) error {
 		called1 = true
 		return nil
-	})
+	}, actr)
 	require.NoError(t, err)
 	require.Equal(t, 1, mImpl.registry.Len(), "second callback not registered")
 

metric/internal/global/meter_types_test.go

@@ -115,7 +115,7 @@ func (m *testMeter) Float64ObservableGauge(name string, options ...instrument.Fl
 //
 // It is only valid to call Observe within the scope of the passed function,
 // and only on the instruments that were registered with this call.
-func (m *testMeter) RegisterCallback(i []instrument.Asynchronous, f metric.Callback) (metric.Registration, error) {
+func (m *testMeter) RegisterCallback(f metric.Callback, i ...instrument.Asynchronous) (metric.Registration, error) {
 	m.callbacks = append(m.callbacks, f)
 	return testReg{
 		f: func(idx int) func() {

metric/meter.go

@@ -102,7 +102,7 @@ type Meter interface {
 	//
 	// If no instruments are passed, f should not be registered nor called
 	// during collection.
-	RegisterCallback(instruments []instrument.Asynchronous, f Callback) (Registration, error)
+	RegisterCallback(f Callback, instruments ...instrument.Asynchronous) (Registration, error)
 }
 
 // Callback is a function registered with a Meter that makes observations for

metric/noop.go

@@ -88,7 +88,7 @@ func (noopMeter) Float64ObservableGauge(string, ...instrument.Float64ObserverOpt
 }
 
 // RegisterCallback creates a register callback that does not record any metrics.
-func (noopMeter) RegisterCallback([]instrument.Asynchronous, Callback) (Registration, error) {
+func (noopMeter) RegisterCallback(Callback, ...instrument.Asynchronous) (Registration, error) {
 	return noopReg{}, nil
 }
 

sdk/metric/meter.go

@@ -200,7 +200,7 @@ func (m *meter) Float64ObservableGauge(name string, options ...instrument.Float6
 
 // RegisterCallback registers the function f to be called when any of the
 // insts Collect method is called.
-func (m *meter) RegisterCallback(insts []instrument.Asynchronous, f metric.Callback) (metric.Registration, error) {
+func (m *meter) RegisterCallback(f metric.Callback, insts ...instrument.Asynchronous) (metric.Registration, error) {
 	for _, inst := range insts {
 		// Only register if at least one instrument has a non-drop aggregation.
 		// Otherwise, calling f during collection will be wasted computation.