Add MeterProvider api (open-telemetry#165)

Author: HudsonHumphries

api/include/opentelemetry/metrics/TBD

@@ -0,0 +1,18 @@
+#pragma once
+#include "opentelemetry/version.h"
+
+OPENTELEMETRY_BEGIN_NAMESPACE
+
+namespace metrics
+{
+
+class Meter
+{
+public:
+  Meter() = default;
+
+};
+
+} // namespace metrics
+
+OPENTELEMETRY_END_NAMESPACE

api/include/opentelemetry/metrics/meter.h

@@ -0,0 +1,27 @@
+#pragma once
+
+#include "opentelemetry/metrics/meter.h"
+#include "opentelemetry/nostd/shared_ptr.h"
+#include "opentelemetry/nostd/string_view.h"
+
+OPENTELEMETRY_BEGIN_NAMESPACE
+namespace metrics
+{
+/**
+ * Creates new Meter instances.
+ */
+class MeterProvider
+{
+public:
+  virtual ~MeterProvider() = default;
+  /**
+   * Gets or creates a named Meter instance.
+   *
+   * Optionally a version can be passed to create a named and versioned Meter
+   * instance.
+   */
+  virtual nostd::shared_ptr<Meter> GetMeter(nostd::string_view library_name,
+                                            nostd::string_view library_version = "") = 0;
+};
+}  // namespace metrics
+OPENTELEMETRY_END_NAMESPACE

api/include/opentelemetry/metrics/meter_provider.h

@@ -0,0 +1,42 @@
+#pragma once
+// Please refer to provider.h for documentation on how to obtain a Meter object.
+//
+// This file is part of the internal implementation of OpenTelemetry. Nothing in this file should be
+// used directly. Please refer to meter.h for documentation on these interfaces.
+
+#include "opentelemetry/metrics/meter.h"
+#include "opentelemetry/metrics/meter_provider.h"
+#include "opentelemetry/nostd/string_view.h"
+#include "opentelemetry/nostd/unique_ptr.h"
+#include "opentelemetry/version.h"
+
+#include <memory>
+
+OPENTELEMETRY_BEGIN_NAMESPACE
+namespace metrics
+{
+/**
+ * No-op implementation of a MeterProvider.
+ */
+class NoopMeter final : public Meter, public std::enable_shared_from_this<NoopMeter> {};
+
+class NoopMeterProvider final : public opentelemetry::metrics::MeterProvider
+{
+public:
+  NoopMeterProvider()
+      : meter_{nostd::shared_ptr<opentelemetry::metrics::NoopMeter>(
+            new opentelemetry::metrics::NoopMeter)}
+  {}
+
+  nostd::shared_ptr<opentelemetry::metrics::Meter> GetMeter(
+      nostd::string_view library_name,
+      nostd::string_view library_version) override
+  {
+    return meter_;
+  }
+
+private:
+  nostd::shared_ptr<opentelemetry::metrics::Meter> meter_;
+};
+}  // namespace metrics
+OPENTELEMETRY_END_NAMESPACE

api/include/opentelemetry/metrics/noop.h

@@ -0,0 +1,60 @@
+#pragma once
+
+#include <atomic>
+
+#include "opentelemetry/metrics/meter_provider.h"
+#include "opentelemetry/metrics/noop.h"
+#include "opentelemetry/nostd/shared_ptr.h"
+
+OPENTELEMETRY_BEGIN_NAMESPACE
+namespace metrics
+{
+/**
+ * Stores the singleton global MeterProvider.
+ */
+class Provider
+{
+public:
+  /**
+   * Returns the singleton MeterProvider.
+   *
+   * By default, a no-op MeterProvider is returned. This will never return a
+   * nullptr MeterProvider.
+   */
+  static nostd::shared_ptr<MeterProvider> GetMeterProvider() noexcept
+  {
+    while (GetLock().test_and_set(std::memory_order_acquire))
+      ;
+    auto provider = nostd::shared_ptr<MeterProvider>(GetProvider());
+    GetLock().clear(std::memory_order_release);
+
+    return provider;
+  }
+
+  /**
+   * Changes the singleton MeterProvider.
+   */
+  static void SetMeterProvider(nostd::shared_ptr<MeterProvider> tp) noexcept
+  {
+    while (GetLock().test_and_set(std::memory_order_acquire))
+      ;
+    GetProvider() = tp; 
+    GetLock().clear(std::memory_order_release);
+  }
+
+private:
+  static nostd::shared_ptr<MeterProvider> &GetProvider() noexcept
+  {
+    static nostd::shared_ptr<MeterProvider> provider(new NoopMeterProvider);
+    return provider;
+  }
+
+  static std::atomic_flag &GetLock() noexcept
+  {
+    static std::atomic_flag lock = ATOMIC_FLAG_INIT;
+    return lock;
+  }
+};
+
+}  // namespace metrics
+OPENTELEMETRY_END_NAMESPACE

api/include/opentelemetry/metrics/provider.h

@@ -2,3 +2,4 @@ add_subdirectory(core)
 add_subdirectory(plugin)
 add_subdirectory(nostd)
 add_subdirectory(trace)
+add_subdirectory(metrics)

api/test/CMakeLists.txt

@@ -0,0 +1,12 @@
+load("//bazel:otel_cc_benchmark.bzl", "otel_cc_benchmark")
+
+cc_test(
+    name = "meter_provider_test",
+    srcs = [
+        "meter_provider_test.cc",
+    ],
+    deps = [
+        "//api",
+        "@com_google_googletest//:gtest_main",
+    ],
+)

api/test/metrics/BUILD

@@ -0,0 +1,6 @@
+foreach(testname meter_provider_test)
+  add_executable(${testname} "${testname}.cc")
+  target_link_libraries(${testname} ${GTEST_BOTH_LIBRARIES}
+                        ${CMAKE_THREAD_LIBS_INIT} opentelemetry_api)
+  gtest_add_tests(TARGET ${testname} TEST_PREFIX metrics. TEST_LIST ${testname})
+endforeach()

api/test/metrics/CMakeLists.txt

@@ -0,0 +1,41 @@
+#include "opentelemetry/metrics/provider.h"
+#include "opentelemetry/nostd/shared_ptr.h"
+
+#include <gtest/gtest.h>
+
+using opentelemetry::metrics::Meter;
+using opentelemetry::metrics::MeterProvider;
+using opentelemetry::metrics::Provider;
+
+class TestProvider : public MeterProvider
+{
+  opentelemetry::nostd::shared_ptr<Meter> GetMeter(
+      opentelemetry::nostd::string_view library_name,
+      opentelemetry::nostd::string_view library_version) override
+  {
+    return opentelemetry::nostd::shared_ptr<Meter>(nullptr);
+  }
+};
+
+TEST(Provider, GetMeterProviderDefault)
+{
+  auto tf = Provider::GetMeterProvider();
+  EXPECT_NE(nullptr, tf);
+}
+
+TEST(Provider, SetMeterProvider)
+{
+  auto tf = opentelemetry::nostd::shared_ptr<MeterProvider>(new TestProvider());
+  Provider::SetMeterProvider(tf);
+  ASSERT_EQ(tf, Provider::GetMeterProvider());
+}
+
+TEST(Provider, MultipleMeterProviders)
+{
+  auto tf = opentelemetry::nostd::shared_ptr<MeterProvider>(new TestProvider());
+  Provider::SetMeterProvider(tf);
+  auto tf2 = opentelemetry::nostd::shared_ptr<MeterProvider>(new TestProvider());
+  Provider::SetMeterProvider(tf2);
+
+  ASSERT_NE(Provider::GetMeterProvider(), tf);
+}