OtlpExporter: HttpClientFactory option

[CodeBlanch]

Closes #2522

Changes

• Adds the same HttpClientFactory option we have on JaegerExporterOptions & ZipkinExporterOptions to OtlpExporterOptions.
• Adds IHttpClientFactory integration when IDeferredTracerProviderBuilder/IDeferredMeterProviderBuilder is used.
• Adds support for the HttpProtobuf protocol in OtlpMetricExporter.

TODOs

• CHANGELOG.md updated for non-trivial changes
• Unit tests

[codecov]

Codecov Report

Merging #2696 (dc3c21a) into main (381e0b8) will increase coverage by 1.09%.
The diff coverage is 91.35%.

@@            Coverage Diff             @@
##             main    #2696      +/-   ##
==========================================
+ Coverage   82.95%   84.05%   +1.09%     
==========================================
  Files         250      251       +1     
  Lines        8705     8778      +73     
==========================================
+ Hits         7221     7378     +157     
+ Misses       1484     1400      -84     

Impacted Files	Coverage Δ
...entation/ExportClient/OtlpHttpTraceExportClient.cs	90.47% <ø> (ø)
...tation/ExportClient/OtlpHttpMetricsExportClient.cs	66.66% <66.66%> (ø)
...mentation/ExportClient/BaseOtlpHttpExportClient.cs	100.00% <100.00%> (+26.08%)	⬆️
...orter.OpenTelemetryProtocol/OtlpExporterOptions.cs	100.00% <100.00%> (ø)
...TelemetryProtocol/OtlpExporterOptionsExtensions.cs	97.80% <100.00%> (+1.80%)	⬆️
...porter.OpenTelemetryProtocol/OtlpMetricExporter.cs	72.72% <100.00%> (+72.72%)	⬆️
...nTelemetryProtocol/OtlpMetricExporterExtensions.cs	100.00% <100.00%> (+100.00%)	⬆️
...metryProtocol/OtlpTraceExporterHelperExtensions.cs	95.00% <100.00%> (+21.31%)	⬆️
src/OpenTelemetry/Metrics/MeterProviderSdk.cs	89.86% <0.00%> (+0.44%)	⬆️
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	74.76% <0.00%> (+0.93%)	⬆️
... and 9 more

[alanwest]

LGTM. Haven't thought it through, but this may actually be of use to the gRPC based exporters as well. At least for compilation that uses the Grpc.Net.Client library, you can set the underlying HttpClient to use. Allows for more sophisticated configuration of the channel. https://grpc.github.io/grpc/csharp-dotnet/api/Grpc.Net.Client.GrpcChannelOptions.html#Grpc_Net_Client_GrpcChannelOptions_HttpClient