Prometheus Exporter HttpListener is unreliable on non-Windows OSes

[pellared]

This issue was found when I was taking another look at the unstable integration tests for Prometheus Exporter (HttpListener) in OTel .NET AutoInstrumentation: open-telemetry/opentelemetry-dotnet-instrumentation#1363

It occurs that there is a bug that the HttpListener can crash when the HTTP requests are sent before it is fully operational. The issue was already reported and closed without a fix: dotnet/runtime#28658.

I suggest that at minimum the docs should say that the Prometheus Exporter HttpListener uses an unmaintained legacy .NET component and it should not be used in production. For this reason, I also propose to move its source code to contrib repo or even remove it completely. One can always use OTLP exporter + OTel collector with Prometheus exporter for debugging. This is our default "dev env setup" in OTel .NET AutoInstrumentation: https://github.com/open-telemetry/opentelemetry-dotnet-instrumentation/blob/main/dev/otel-config.yaml

CC @reyang @Kielek

[reyang]

I suggest that at minimum the docs should say that the Prometheus Exporter HttpListener uses an unmaintained legacy .NET component and it should not be used in production. For this reason, I also propose to move its source code to contrib repo or even remove it completely. One can always use OTLP exporter + OTel collector with Prometheus exporter for debugging. This is our default "dev env setup" in OTel .NET AutoInstrumentation: https://github.com/open-telemetry/opentelemetry-dotnet-instrumentation/blob/main/dev/otel-config.yaml

+1 on clarifying that Prometheus Exporter HttpListener is currently a dev-inner-loop offering (until we can actually make it production ready) #3694 (review). I don't have strong opinion whether it should move to the contrib repo or not.

[cijothomas]

I did some basic search to find if there is any official guidance about httplistener being prod-grade, now this is a good reason to call out that this component is not meant for production scenarios.

[pellared]

A quick note before I go asleep and forget (the note may not be the best quality but better than nothing)

I think that an exception could be thrown from here:

opentelemetry-dotnet/src/OpenTelemetry.Exporter.Prometheus.HttpListener/PrometheusHttpListener.cs

Lines 125 to 127 in e826f9d

	var ctxTask = this.httpListener.GetContextAsync();
	ctxTask.Wait(this.tokenSource.Token);
	var ctx = ctxTask.Result;

which would lead to the HttpListener shutdown.

1. I suggest to use var ctx = this.httpListener.GetContext(); instead. There is no real benefit of using Async here (makes it just more error prone).
2. I think this fragment should have a try_catch

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