Asynchronous Aggregation storage

[lalitb]

Fixes #1218

Changes

1. Implementation for ObserverResult to be called by instrumentation library to push the measurements asynchronously.
2. Asynchronous Storage implementation
   • During metric collection, the storage invokes the configured measurement callback, which enables the instrumentation library/application to call the ObserverResult::Observe() method and push the measurement.
   • The measurement is filtered and processed to generate the metrics.

The flow of measurement for Asynchronous instrument would be:

Metric Collection Operation -> AsynMetricStorage::Collect() -> measurement_callback(observer_result) -> ObserverResult::Observe() -> AttributeProcessor::Process() -> Aggregation::Aggregate() -> AttributeHashMap::Insert()

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1232 (a45f968) into main (4a7200b) will increase coverage by 0.06%.
The diff coverage is 77.91%.

@@            Coverage Diff             @@
##             main    #1232      +/-   ##
==========================================
+ Coverage   92.31%   92.37%   +0.06%     
==========================================
  Files         198      202       +4     
  Lines        7281     7335      +54     
==========================================
+ Hits         6721     6775      +54     
  Misses        560      560              

Impacted Files	Coverage Δ
.../opentelemetry/sdk/metrics/measurement_processor.h	0.00% <ø> (ø)
...etry/sdk/metrics/aggregation/default_aggregation.h	14.71% <27.78%> (+14.71%)	⬆️
...ntelemetry/sdk/metrics/state/sync_metric_storage.h	67.75% <50.00%> (+13.90%)	⬆️
...telemetry/sdk/metrics/state/async_metric_storage.h	92.86% <92.86%> (ø)
...nclude/opentelemetry/sdk/metrics/observer_result.h	100.00% <100.00%> (ø)
sdk/test/metrics/async_metric_storage_test.cc	100.00% <100.00%> (ø)
sdk/test/metrics/observer_result_test.cc	100.00% <100.00%> (ø)
sdk/test/metrics/sync_metric_storage_test.cc	100.00% <100.00%> (ø)

[esigo]

metrics.AsyncMetricStorageTest.BasicTests is failing because of timeout.

[lalitb]

metrics.AsyncMetricStorageTest.BasicTests is failing because of timeout.

Thanks, it was issue in the test code, have fixed it now.

[esigo]

LGTM
Thanks for the PR :)