Custom Aggregation support

[lalitb]

Fixes #1664

Changes

This is ready to review now. With the change, it is possible to use the custom aggregator for the Instruments.
Also added tests for:

• Counter Instrument to Sum Aggregation (this is default)
• Histogram Instrument to Sum Aggregation
• Last Value Instrument to Sum Aggregation
• Histogram Instrument to Histogram Aggregation (this is default)
• Counter Instrument to Histogram Aggregation

As a side change, it fixes few warning for unused variables.

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 #1899 (d28ba85) into main (e8fce84) will increase coverage by 0.23%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1899      +/-   ##
==========================================
+ Coverage   86.90%   87.12%   +0.23%     
==========================================
  Files         165      165              
  Lines        4594     4596       +2     
==========================================
+ Hits         3992     4004      +12     
+ Misses        602      592      -10     

Impacted Files	Coverage Δ
...etry/sdk/metrics/aggregation/default_aggregation.h	72.59% <100.00%> (-6.10%)	⬇️
...telemetry/sdk/metrics/state/async_metric_storage.h	86.12% <100.00%> (ø)
...ntelemetry/sdk/metrics/state/sync_metric_storage.h	87.50% <100.00%> (+0.41%)	⬆️
sdk/src/metrics/meter.cc	61.22% <100.00%> (+7.00%)	⬆️
sdk/src/metrics/state/temporal_metric_storage.cc	98.25% <100.00%> (+0.04%)	⬆️
...include/opentelemetry/sdk/common/circular_buffer.h	100.00% <0.00%> (+2.00%)	⬆️
...telemetry/sdk/metrics/state/multi_metric_storage.h	83.34% <0.00%> (+3.34%)	⬆️

[lalitb]

@mprzybylski Have created a draft PR, if you want to test this. Haven't added all unit-test scenarios, basically blocked on #1869 to be merged before adding tests for histogram aggregation. But should be good enough to test.

[esigo]

LGTM
just had one nit comment.
Thanks for the PR :)