Fix observable Gauge metrics generation

[lalitb]

Fixes #1588

Changes

While #1641 fixes the observable instrument metrics generation in general, the gauge instrument was broken separately. This PR fixes the metrics generation for gauge instrument by

• ensuring sampling timestamp is correctly updated during aggregation.
• ensuring the correct aggregation type ( double or long) is returned during merge/diff

Test cases are added.

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 #1651 (6b480ee) into main (a64ac09) will increase coverage by 0.54%.
The diff coverage is 33.34%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1651      +/-   ##
==========================================
+ Coverage   84.35%   84.88%   +0.54%     
==========================================
  Files         170      170              
  Lines        5130     5132       +2     
==========================================
+ Hits         4327     4356      +29     
+ Misses        803      776      -27     

Impacted Files	Coverage Δ
...k/src/metrics/aggregation/lastvalue_aggregation.cc	66.67% <33.34%> (+29.17%)	⬆️
sdk/src/trace/batch_span_processor.cc	90.63% <0.00%> (-0.78%)	⬇️
...etry/sdk/metrics/aggregation/default_aggregation.h	76.37% <0.00%> (+18.19%)	⬆️

[esigo]

LGTM
Thanks for the fix :)