Fix prometheus exporter to handle double values

[cijothomas]

Will do cleanups/extracting common logic to reusable functions later. This PR just addresses missing functionality.

[cijothomas]

unrelated, but required fix as metrics branch got broken with the latest main->metrics merge and conflict resolution.

[codecov]

Codecov Report

Merging #2317 (2b5623c) into metrics (de5f0a6) will increase coverage by 0.04%.
The diff coverage is 90.00%.

@@             Coverage Diff             @@
##           metrics    #2317      +/-   ##
===========================================
+ Coverage    77.87%   77.91%   +0.04%     
===========================================
  Files          240      241       +1     
  Lines         7565     7617      +52     
===========================================
+ Hits          5891     5935      +44     
- Misses        1674     1682       +8     

Impacted Files	Coverage Δ
src/OpenTelemetry/CompositeProcessor.cs	92.95% <ø> (ø)
...ourceInstrumentation/DiagnosticSourceSubscriber.cs	95.00% <ø> (ø)
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	70.65% <57.14%> (-1.12%)	⬇️
...metry/Trace/BatchExportActivityProcessorOptions.cs	83.33% <83.33%> (ø)
src/OpenTelemetry.Api/Baggage.cs	100.00% <100.00%> (ø)
...Telemetry.Exporter.Jaeger/JaegerExporterOptions.cs	84.21% <100.00%> (ø)
...orter.OpenTelemetryProtocol/OtlpExporterOptions.cs	88.46% <100.00%> (ø)
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	100.00% <100.00%> (ø)
...Telemetry.Exporter.Zipkin/ZipkinExporterOptions.cs	100.00% <100.00%> (ø)
...ZPages/Implementation/ZPagesExporterEventSource.cs	56.25% <0.00%> (-6.25%)	⬇️
... and 1 more