Add Histogram tests for Synchronous Metric Storage.

[lalitb]

Changes

The histogram tests were missing for delta and cumulative.

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 #1597 (e830714) into main (aedd610) will increase coverage by 0.29%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1597      +/-   ##
==========================================
+ Coverage   84.98%   85.26%   +0.29%     
==========================================
  Files         156      156              
  Lines        4978     4978              
==========================================
+ Hits         4230     4244      +14     
+ Misses        748      734      -14     

Impacted Files	Coverage Δ
sdk/src/trace/batch_span_processor.cc	91.41% <0.00%> (+0.79%)	⬆️
ext/src/http/client/curl/http_client_curl.cc	81.44% <0.00%> (+1.14%)	⬆️
...etry/sdk/metrics/aggregation/default_aggregation.h	58.19% <0.00%> (+18.19%)	⬆️

[esigo]

LGTM
Thanks for the PR :)