[Exporter.Geneva][otlp protobuf] Add support for Gauge

[vishweshbankwar]

Towards #1585

Changes

Support for Gauge instruments and refactoring to use common method for serializing datapoints.

For significant contributions please make sure you have completed the following items:

• Appropriate CHANGELOG.md updated for non-trivial changes
• Design discussion issue #
• Changes in public API reviewed

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 60.01%. Comparing base (71655ce) to head (6ad8c79).
Report is 176 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1634       +/-   ##
===========================================
- Coverage   73.91%   60.01%   -13.91%     
===========================================
  Files         267       35      -232     
  Lines        9615     3381     -6234     
===========================================
- Hits         7107     2029     -5078     
+ Misses       2508     1352     -1156     

Flag	Coverage Δ
unittests-Exporter.Geneva	60.01% <90.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...eva/Metrics/OtlpProtobuf/OtlpProtobufSerializer.cs	88.29% <90.00%> (ø)

... and 243 files with indirect coverage changes