[feature request] Allow sending telemetry in HTTP2

[NatMarchand]

Package

OpenTelemetry.Exporter.OpenTelemetryProtocol

Is your feature request related to a problem?

It is currently impossible to force OtlpExporter to use HTTP2
Using OpenTelemetry 1.12 with OtlpExporter, we are using a delegating handler in the HttpClientFactory to enforce the version of the HttpRequestMessage to HTTP2:

POST http://opentelemetry-collector-sts-collector.monitoring.svc.cluster.local/v1/metrics
      System.NotSupportedException: The synchronous method is not supported by 'System.Net.Http.SocketsHttpHandler' for HTTP/2 or higher. Either use an asynchronous method or downgrade the request version to HTTP/1.1 or lower.
         at System.Net.Http.SocketsHttpHandler.Send(HttpRequestMessage request, CancellationToken cancellationToken)

What is the expected behavior?

We are expecting to be able to enable HTTP2 on all the requests (and thus use the Async counterpart of Send)

Which alternative solutions or features have you considered?

We are currently removing the DelegatingHandler for this HttpClient

Additional context

No response

[martincostello]

Looking at the code, SendAsync() is used when RequireHttp2 is set to true:

opentelemetry-dotnet/src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/ExportClient/OtlpExportClient.cs

Lines 94 to 105 in 8321588

	protected HttpResponseMessage SendHttpRequest(HttpRequestMessage request, CancellationToken cancellationToken)
	{
	#if NET
	// Note: SendAsync must be used with HTTP/2 because synchronous send is
	// not supported.
	return this.RequireHttp2 || !SynchronousSendSupportedByCurrentPlatform
	? this.HttpClient.SendAsync(request, cancellationToken).GetAwaiter().GetResult()
	: this.HttpClient.Send(request, cancellationToken);
	#else
	return this.HttpClient.SendAsync(request, cancellationToken).GetAwaiter().GetResult();
	#endif
	}

This is only set to true when using OtlpGrpcExportClient:

opentelemetry-dotnet/src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/ExportClient/OtlpGrpcExportClient.cs

Line 34 in 8321588

internal override bool RequireHttp2 => true;

Supporting this would need some sort of new opt-in setting to allow OtlpHttpExportClient (or other) implementations to request use of HTTP/2.

[NatMarchand]

Would it be problematic to always use SendAsync ? Or is there any perf penalty ?

[martincostello]

I don't know the background on why Send() is being used where available. I assume it's a performance optimization to avoid the background thread sending the metrics needing to allocate the async state machinery.

[NatMarchand]

After digging a moment, I've found : #2316

It seems that as the BaseExporter signatures are synchronous, it was chosen to pick the Send method (or use the SendAsync with sync over async in the net5 case)