OTLP exporter improvements

[alanwest]

Currently the constructor for the OtlpExporter instantiates a gRPC channel here:

opentelemetry-dotnet/src/OpenTelemetry.Exporter.OpenTelemetryProtocol/OtlpExporter.cs

Line 59 in 1630fb3

this.channel = new Channel(options.Endpoint, options.Credentials, options.ChannelOptions);

The channel constructor can throw an exception. I've encountered this when using a single file publish:

dotnet publish -r linux-x64 -p:PublishSingleFile=true

Using the single file publish causes this issue in the gRPC library grpc/grpc#24266.

#1616 also suggests that the OtlpExporter constructor is throwing an excption. I'm uncertain if it is the same one, but it is most likely the instantiation of the channel that is throwing here too.

Few things I'd like to suggest:

1. Delay the instantiation of the channel to the first call to export data. This would allow us to catch the exception on export and not crash the application.
2. Make a netstandard2.1 build of the OTLP exporter. This would enable us to use the grpc-dotnet library for .NET 3.1 and above instead of the native grpc library. This would:
   • Resolve the issue with single file publish
   • Provide a better fix for Can't use OtlpExporter in an Alpine docker image #1251 - which has a work around, but has some dubious security concerns C#: Running in Alpine Linux docker container fails with "Grpc.Core.Internal.UnmanagedLibrary Attempting to load native library "/app/libgrpc_csharp_ext.x64.so" grpc/grpc#21446 (comment)
   • And may help AddOtlpExporter seems to be causing a crash? #1616 - though I'm not entirely certain of this yet...