Make UseOpenTelemetryPrometheusScrapingEndpoint a real endpoint

[JamesNK]

Feature Request

UseOpenTelemetryPrometheusScrapingEndpoint is middleware that provides an endpoint for Prometheus scraping.

This kind of feature is usually provided by an endpoint using ASP.NET Core endpoint routing. For example, MapHealthChecks configures the health probe endpoint. See https://learn.microsoft.com/en-us/aspnet/core/host-and-deploy/health-checks?view=aspnetcore-7.0#basic-health-probe.

An endpoint allows the feature to more easily hook into other endpoint-aware middleware and routing features. Also, routing is very fast, and the check for the scraping path can happen at the same time as routing instead of extra overhead.

Describe the solution you'd like:

Add a MapPrometheusScraping extension method for this task.

[CodeBlanch]

@JamesNK We also have these extensions: https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/src/OpenTelemetry.Exporter.Prometheus.AspNetCore/PrometheusExporterEndpointRouteBuilderExtensions.cs

Is what you are asking for something different?

[klinki]

Hi, we are currently considering using this library instead of prometheus-net and we run into issues with endpoints not being properly exposed for further modifications. Pretty much what @JamesNK was referring to.

What we need is to add custom authentication.

With prometheus-net we use this:

		app.UseEndpoints(endpoints =>
		{
			endpoints.MapMetrics(url).RequireAuthorization(PrometheusAuth.Policy);
		});

which is pretty easy.

MapMetrics exposes IEndpointConventionBuilder which is easy to customize.

You can check source code here: https://github.com/prometheus-net/prometheus-net/blob/master/Prometheus.AspNetCore/MetricServerMiddlewareExtensions.cs#L17

Adding MapPrometheusScraping method similar to the prometheus-net one would be really great.

I could make a PR if you would be interested in it.

[klinki]

Oh, it is there I just didn't notice.

But I have found it now - https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/src/OpenTelemetry.Exporter.Prometheus.AspNetCore/PrometheusExporterEndpointRouteBuilderExtensions.cs

So you can ignore my previous comment :)

[JamesNK]

I think this is already done with MapPrometheusScrapingEndpoint. I didn't see this before creating the issue.

[rmannibucau]

Hi,

AFAIK there is a gap between use and map flavors: the use enables to not enable the endpoint when on a different URL (very important for kubernetes where you rarely - never? - want to bind the endpoint on the main URL which is forwarded to services+ingresses for ex.

However the coming feature to disable http metrics (dotnet/aspnetcore#56036) will not be available to the middleware directly and will need a hack to be enabled.

So overall I think it would be great to merge both API in terms of capabilities - in particular when the impl is pretty close.

Goals (to get by default using the minimal api):

1. Be able to expose the endpoint only on some url/server and not globally
2. Be able to ignore the metrics related to the opentelemetry endpoints/pipelines

Side note: this can be related to other endpoints - health, tracing, ... - didn't review them all.

Is it possible to reopen and fix even if it requires v9 to be fully functional?