Add OpenTelemetry Meter API

[Brandon-Kimberly]

I am working on implementing the metrics API and SDK for C++. This is one of a series of PRs I plan to file along with @ankit-bhargava and @HudsonHumphries.

This is a cut at implementing the API's Meter Class defined in the specification. This class provides functions to create and return all metric instrument types. It also provides a function to record to a batch of instruments in a single API call.

I have also included the default no-op implementation of the meter class in metrics/noop.h. The default no-op MeterProvider will instantiate this class. This class is only capable of creating and returning no-op metric instruments.

Lastly, I have included a no-op meter test. This is similar to the no-op tracer test and checks that there are no errors when creating metric instruments and calling BatchRecord.

Note: This PR does not currently build. The exact reason why is listed in the review section below.

[Brandon-Kimberly]

I have updated the meter class to address some feedback left on this PR and to improve integration with other metrics classes. It is ready to re-review.

NOTE:
However, this PR is still not ready to merge as the RecordBatch test fails for a similar reason listed in PR #212 and expounded up in issue #216. The nostd::shared_ptr class does not implement std::static_pointer_cast and therefore the implicit upcasts from a counter instrument to SynchronousInstrument in the RecordBatch functions do not work correctly. Once nostd::shared_ptr can implicitly upcast then this test will pass and this PR should be ready to merge.

[Brandon-Kimberly]

Hello all, I have updated the Meter class to remove the need to use dynamic_pointer_cast. However, this requires working with raw pointers in the RecordBatch function. This is not ideal but it is required due to limitations of nostd::shared_ptr.

This PR does not currently pass CI tests due to an issue in the MeterProvider class that was already merged. However, once that is fixed this PR will be ready to merge, pending final approval.

[codecov]

Codecov Report

Merging #162 into master will decrease coverage by 2.03%.
The diff coverage is 43.29%.

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
- Coverage   94.22%   92.19%   -2.04%     
==========================================
  Files         115      117       +2     
  Lines        3882     4038     +156     
==========================================
+ Hits         3658     3723      +65     
- Misses        224      315      +91     

Impacted Files	Coverage Δ
sdk/include/opentelemetry/sdk/metrics/meter.h	6.66% <3.44%> (-93.34%)	⬇️
api/include/opentelemetry/metrics/noop.h	45.03% <47.54%> (-0.89%)	⬇️
api/test/metrics/noop_metrics_test.cc	88.63% <88.63%> (ø)
api/include/opentelemetry/metrics/meter.h	100.00% <100.00%> (ø)
sdk/src/metrics/meter_provider.cc	100.00% <0.00%> (ø)

[Brandon-Kimberly]

@reyang This PR is ready to merge, pending final approval. I had to add a stubbed sdk/meter class so that the MeterProvider test would pass. This is only a stubbed version and will be overwritten when PR #212 is merged.

[reyang]

LGTM.

[Brandon-Kimberly]

@reyang I believe this PR is ready to merge :)