Make summary quantiles non-const

This change removes an unnecessary API constraint that makes it
harder to dynamically construct quantile configurations for summaries.

Fixes #425

core/include/prometheus/detail/ckms_quantiles.h

@@ -15,21 +15,21 @@ namespace detail {
 class PROMETHEUS_CPP_CORE_EXPORT CKMSQuantiles {
  public:
   struct PROMETHEUS_CPP_CORE_EXPORT Quantile {
-    const double quantile;
-    const double error;
-    const double u;
-    const double v;
-
     Quantile(double quantile, double error);
+
+    double quantile;
+    double error;
+    double u;
+    double v;
   };
 
  private:
   struct Item {
-    /*const*/ double value;
+    double value;
     int g;
-    /*const*/ int delta;
+    int delta;
 
-    explicit Item(double value, int lower_delta, int delta);
+    Item(double value, int lower_delta, int delta);
   };
 
  public:

core/tests/summary_test.cc

@@ -2,6 +2,7 @@
 
 #include <gtest/gtest.h>
 
+#include <chrono>
 #include <cmath>
 #include <limits>
 #include <memory>
@@ -92,5 +93,13 @@ TEST(SummaryTest, max_age) {
   test_value(std::numeric_limits<double>::quiet_NaN());
 }
 
+TEST(SummaryTest, construction_with_dynamic_quantile_vector) {
+  auto quantiles = Summary::Quantiles{{0.99, 0.001}};
+  quantiles.push_back({0.5, 0.05});
+
+  Summary summary{quantiles, std::chrono::seconds(1), 2};
+  summary.Observe(8.0);
+}
+
 }  // namespace
 }  // namespace prometheus