After update to 1.8 version and when using the ASP .Net Core Prometheus exporter and /metrics end point I get error on Prometheus some times.

[jnormen]

Bug Report

List of all OpenTelemetry NuGet
packages and version that you are
using (e.g. OpenTelemetry 1.0.2):

Metrics I use:
.AddMeter("Microsoft.AspNetCore.Hosting",
"Microsoft.AspNetCore.Routing",
"Microsoft.AspNetCore.Diagnostics",
"Microsoft.AspNetCore.Server.Kestrel",
"Polly",
"MassTransit"

Symptom

PrometheusTargetMissing ***** (http monitor/prometheus critical)[FIRING:3]

With info:
A Prometheus target has disappeared. An exporter might be crashed.

The text mention:
"expected equal, got INVALID"

When I revert to version 1.7 the problem stops with the same meters on.

Same error in beta and rc1 version of 1.8 exporter. Not a valid output.

What did you see instead?

I get alarm from Prometheus with:
description = A Prometheus target has disappeared. An exporter might be crashed. VALUE = 0 LABELS =

Reproduce

Use 1.8 version of Open Telemetry and the 1.8 rc1 of the Prometheus asp.net core exporter

[Abrynos]

This issue is caused by Prometheus adding the Accept: application/openmetrics-text header. I've got this reproducible with both Prometheus version 2.45.3+ds-3 (currently in Debian testing) and with

curl -H "Accept: application/openmetrics-text" {protocol}://{host}:{port}/{configured_endpoint} # incorrect response body

In comparison, the response comes out flawlessly using either command-line below or OpenTelemetry.Exporter.Prometheus.AspNetCore version 1.7.0-rc1 or earlier

curl {protocol}://{host}:{port}/{configured_endpoint} # works

Here's a sample response camptured via curl: test.txt

---

Related PR: JustArchiNET/ArchiSteamFarm#3183

Additional thanks to @ezhevita for investing your time as well.

[martinjt]

I believe this the same as #5515, which is being fixed. If that seems right, we can close this in favour of that.

[reyang]

Duplicated with #5515. The bug was introduced by #5107 and fixed by #5623.