[sdk-metrics] Support .NET 9 Synchronous Gauge

[rajkumar-rangaraj]

Fixes #4805
Design discussion issue #

Changes

Please provide a brief description of the changes here.

Updated the Metric class to add support for synchronous gauge.

Follow up: Update document - #5868

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.28%. Comparing base (6250307) to head (8539901).
Report is 333 commits behind head on main.

Files with missing lines	Patch %	Lines
src/OpenTelemetry/Metrics/Metric.cs	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5867      +/-   ##
==========================================
+ Coverage   83.38%   86.28%   +2.89%     
==========================================
  Files         297      257      -40     
  Lines       12531    11214    -1317     
==========================================
- Hits        10449     9676     -773     
+ Misses       2082     1538     -544     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	86.26% <81.81%> (?)
unittests-Project-Stable	85.95% <81.81%> (?)

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

Files with missing lines	Coverage Δ
src/OpenTelemetry/Metrics/Reader/MetricReader.cs	87.82% <100.00%> (ø)
src/OpenTelemetry/Metrics/Metric.cs	96.89% <80.00%> (+0.37%)	⬆️

... and 231 files with indirect coverage changes

[reyang]

LGTM, would you also update the docs? For example

opentelemetry-dotnet/docs/metrics/README.md

Line 89 in cc0b9e9

| [Gauge](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#gauge) (experimental) | N/A |

[Kielek]

Q: Do you know if it should trigger any changes on instrumentation (packages) side?
Contrib repository/natively instrumented packages?

[cijothomas]

Change looks good.
Requesting changes to ensure the test covers a corner case with Gauge and Temporality, see https://github.com/open-telemetry/opentelemetry-dotnet/pull/5867/files#r1778773598

[rajkumar-rangaraj]

Q: Do you know if it should trigger any changes on instrumentation (packages) side? Contrib repository/natively instrumented packages?

Not yet, need to investigate it.