Avoid copy MetricPoint in exporter

[cijothomas]

Actually applying changes from #2321 to each exporters.

[codecov]

Codecov Report

Merging #2338 (2d471c7) into main (3e0c451) will not change coverage.
The diff coverage is 20.00%.

@@           Coverage Diff           @@
##             main    #2338   +/-   ##
=======================================
  Coverage   80.65%   80.65%           
=======================================
  Files         229      229           
  Lines        7304     7304           
=======================================
  Hits         5891     5891           
  Misses       1413     1413           

Impacted Files	Coverage Δ
...tryProtocol/Implementation/MetricItemExtensions.cs	39.52% <20.00%> (ø)
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	76.47% <0.00%> (-5.89%)	⬇️
...ZPages/Implementation/ZPagesExporterEventSource.cs	62.50% <0.00%> (+6.25%)	⬆️
...Zipkin/Implementation/ZipkinExporterEventSource.cs	72.72% <0.00%> (+9.09%)	⬆️