[OTLP Exporter] Configurable gRPC ChannelOptions

[pcwiese]

We need to allow users to configure channel options for the OTLP exporter.

Another option here is to ditch the various options specific to creating a channel and just expose Func<Channel> in the options. Let consumers do whatever they want when constructing the Channel. I might like that better...

[codecov]

Codecov Report

Merging #1033 into master will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1033      +/-   ##
==========================================
- Coverage   77.12%   77.06%   -0.07%     
==========================================
  Files         221      221              
  Lines        6155     6156       +1     
==========================================
- Hits         4747     4744       -3     
- Misses       1408     1412       +4     

Impacted Files	Coverage Δ
...try.Exporter.OpenTelemetryProtocol/OtlpExporter.cs	50.00% <100.00%> (ø)
...orter.OpenTelemetryProtocol/OtlpExporterOptions.cs	100.00% <100.00%> (ø)
...us/Implementation/PrometheusExporterEventSource.cs	63.63% <0.00%> (-9.10%)	⬇️
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	82.35% <0.00%> (-4.42%)	⬇️

[CodeBlanch]

Another option here is to ditch the various options specific to creating a channel and just expose Func in the options. Let consumers do whatever they want when constructing the Channel. I might like that better...

@pcwiese I'm not at all familiar with gRPC. Are there many more options than what we currently expose? I kind of like the Func idea, especially if there are a lot of other things that can be configured.

Aso, we should probably note in CHANGELOG.

[pcwiese]

There are a ton of options and the way this PR is passing them through is completely fine as is. Take a look at https://grpc.github.io/grpc/cpp/grpc__types_8h.html for the entire defined set.

Each channel option is just setting one of the GRPC_ARG* values