[Metrics SDK] Add support for Pull Metric Exporter

[sam-leitch-oxb]

The MeterProvider::AddMetricReader interface does not make it easy to create pull-based readers. For example, my unit tests look something like this:

// Attempt at Pull-based test reader
class TestReader : public opentelemetry::v1::sdk::metrics::MetricReader {
  void OnInitialized() noexcept override {}

  bool OnForceFlush(std::chrono::microseconds timeout) noexcept override {
    return true;
  }

  bool OnShutDown(std::chrono::microseconds timeout) noexcept override {
    return true;
  }
};

BOOST_AUTO_TEST_CASE(TestMetrics) {
  opentelemetry::v1::sdk::metrics::MeterProvider meter_provider;
  auto meter = meter_provider.GetMeter("unit_test_meter", "0.0.0");
  auto reader = std::make_unique<TestReader>();
  auto* readerPtr = reader.get();  // Work-around for funky OTel SDK
  meter_provider.AddMetricReader(std::move(reader));

  // ... generate metric data ...

  readerPtr->Collect([&](auto& records) {
    BOOST_TEST(has(records, "my.test.meter", {{"att1", "value1"}}, 12345L));

    return true;
  });
}

Notice the raw pointer reference to object owned by the std::unique_ptr (yuck)

I would suggest two options for an alternative:

The simplest change would be to change the AddMetricReader method:

  void AddMetricReader(std::shared_ptr<MetricReader> reader) noexcept;

To allow for shared ownership.

A more complex (and somewhat nicer IMHO) fix would be to abstract away the MetricReader class entirely:

  void AddPeriodicPushMetricExporter(std::unique_ptr<MetricExporter> exporter, std::chrono::duration interval, std::chrono::duration timeout);

  std::unique_ptr<MetricReader> AddPullMetricExporter(std::unique_ptr<MetricExporter> exporter, std::chrono::duration timeout);

Where the returned MetricReader interface provides the Collect() method that can be called to trigger MetricExporter::Collect

Both options follow the SDK documentation as well as I understand them.

[lalitb]

Thanks for raising the issue. I like the second approach, though it needs some design discussion. As we are nearing the Release Candidate, have added it for the next milestone (GA).