METRICS SDK - Calling Observable Instruments callback during metrics collection

[lalitb]

Fixes #1550, #1556

Changes

Please provide a brief description of the changes here.

1. Create ObservableRegistry during Meter creation.
2. Store the reference to ObservableRegistry while Instruments created from that Meter.
3. During the collection (in Meter::Collect()), call all the registered callbacks once.

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 #1554 (7a46d43) into main (d04e1ce) will increase coverage by 1.64%.
The diff coverage is 54.55%.

@@            Coverage Diff             @@
##             main    #1554      +/-   ##
==========================================
+ Coverage   83.38%   85.02%   +1.64%     
==========================================
  Files         156      156              
  Lines        4975     4977       +2     
==========================================
+ Hits         4148     4231      +83     
+ Misses        827      746      -81     

Impacted Files	Coverage Δ
sdk/src/metrics/meter.cc	33.08% <44.45%> (+28.28%)	⬆️
sdk/src/metrics/async_instruments.cc	80.00% <100.00%> (+26.67%)	⬆️
...etry/sdk/metrics/aggregation/default_aggregation.h	40.00% <0.00%> (+7.28%)	⬆️
sdk/src/metrics/meter_context.cc	57.15% <0.00%> (+14.29%)	⬆️
...telemetry/sdk/metrics/state/multi_metric_storage.h	80.00% <0.00%> (+23.34%)	⬆️
sdk/src/metrics/state/metric_collector.cc	76.00% <0.00%> (+28.00%)	⬆️
sdk/include/opentelemetry/sdk/metrics/view/view.h	100.00% <0.00%> (+36.37%)	⬆️
sdk/src/metrics/state/observable_registry.cc	57.15% <0.00%> (+42.86%)	⬆️

[esigo]

is this finished?

[lalitb]

is this finished?

Yes changes are fine, as I tested with ostream exporter, I need to add unit test. Will be finishing it today.

[lalitb]

is this finished?

Yes changes are fine, as I tested with ostream exporter, I need to add unit test. Will be finishing it today.

I have now added basic tests to validate the async flow, it should be good to review now.

[esigo]

LGTM
Thanks for the fix.