Support multiple async callbacks

[lalitb]

Fixes #1338

Changes

Please provide a brief description of the changes here.

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 #1495 (f4b6b37) into main (d299348) will decrease coverage by 0.86%.
The diff coverage is 25.95%.

@@            Coverage Diff             @@
##             main    #1495      +/-   ##
==========================================
- Coverage   84.57%   83.72%   -0.85%     
==========================================
  Files         156      156              
  Lines        4795     4850      +55     
==========================================
+ Hits         4055     4060       +5     
- Misses        740      790      +50     

Impacted Files	Coverage Δ
api/include/opentelemetry/metrics/meter.h	100.00% <ø> (ø)
api/include/opentelemetry/metrics/noop.h	36.00% <0.00%> (-3.13%)	⬇️
...dk/include/opentelemetry/sdk/metrics/instruments.h	0.00% <0.00%> (ø)
...ntelemetry/sdk/metrics/state/sync_metric_storage.h	64.71% <ø> (ø)
...clude/opentelemetry/sdk/metrics/sync_instruments.h	100.00% <ø> (ø)
sdk/src/metrics/meter.cc	5.27% <0.00%> (-1.05%)	⬇️
sdk/src/metrics/state/observable_registry.cc	15.16% <15.16%> (ø)
...telemetry/sdk/metrics/state/multi_metric_storage.h	56.67% <21.43%> (-31.56%)	⬇️
sdk/src/metrics/async_instruments.cc	53.34% <53.34%> (ø)
...telemetry/sdk/metrics/state/async_metric_storage.h	85.30% <73.69%> (+1.97%)	⬆️
... and 9 more

[esigo]

Thanks for the RP.
I had some minor comments.

[esigo]

LGTM
Thanks for the PR :)