Make MetricPoint access from MetricPointAccessor readonly

[utpilla]

Fixes the issue described in #2731

Changes

• Make MetricPoint access from MetricPointAccessor readonly

Please provide a brief description of the changes here.

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

Merging #2736 (20a1126) into main (83fa9fd) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2736      +/-   ##
==========================================
+ Coverage   83.73%   83.76%   +0.02%     
==========================================
  Files         250      250              
  Lines        8824     8824              
==========================================
+ Hits         7389     7391       +2     
+ Misses       1435     1433       -2     

Impacted Files	Coverage Δ
src/OpenTelemetry/Metrics/MetricPointsAccessor.cs	100.00% <ø> (ø)
...tryProtocol/Implementation/MetricItemExtensions.cs	93.52% <100.00%> (ø)
...ometheus/Implementation/PrometheusSerializerExt.cs	100.00% <100.00%> (ø)
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	79.41% <0.00%> (-2.95%)	⬇️
src/OpenTelemetry/BatchExportProcessor.cs	89.13% <0.00%> (+3.26%)	⬆️

[cijothomas]

Missing changelog. Will do as a separate follow up PR.