Prometheus not Scraping Custom Metrics (prometheus_demo_request_total) from ASP.NET Core Application with OpenTelemetry

Question

I have implemented a custom middleware in my ASP.NET Core application to log HTTP request metrics using Prometheus. I implemented the middleware using the link. However, despite the middleware being triggered for each HTTP request, these custom metrics are not appearing when accessing the /metrics endpoint.

I have confirmed that the counter.Labels(...).Inc() calls are executed during the middleware's invocation, but the expected metrics are missing in the Prometheus metrics output. Other default metrics from OpenTelemetry are present.

I seek assistance in identifying the root cause of this issue and ensuring that the custom metrics are correctly scraped and displayed by Prometheus.

My Startup.cs class looks like this:

public void ConfigureServices(IServiceCollection services)
{
    ...

    services.AddOpenTelemetry()
                .WithMetrics(builder => builder
                    .AddRuntimeInstrumentation()
                    .AddAspNetCoreInstrumentation()
                    .AddHttpClientInstrumentation()
                    .AddPrometheusExporter());
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env, CustomDbContext dbContext, ILogger<RedactableDbContext> logger)
{
    app.UseRequestMiddleware();

    ...
}

The following are the libraries that I am using for OpenTelemetry and Prometheus:

<PackageReference Include="OpenTelemetry.Exporter.Console" Version="1.6.0" />
<PackageReference Include="OpenTelemetry.Exporter.OpenTelemetryProtocol" Version="1.6.0" />
<PackageReference Include="OpenTelemetry.Exporter.Prometheus.AspNetCore" Version="1.6.0-rc.1" />
<PackageReference Include="OpenTelemetry.Extensions.Hosting" Version="1.6.0" />
<PackageReference Include="OpenTelemetry.Instrumentation.AspNetCore" Version="1.5.1-beta.1" />
<PackageReference Include="OpenTelemetry.Instrumentation.Http" Version="1.5.1-beta.1" />
<PackageReference Include="OpenTelemetry.Instrumentation.RunTime" Version="1.5.1" />
<PackageReference Include="prometheus-net.AspNetCore" Version="8.2.0" />

Answer 1

Custom metrics need to be registered with the OpenTelemetry SDK.

Modify your existing setup code to include a call to AddMeter:

services.AddOpenTelemetry()
    .WithMetrics(builder => builder
        .AddRuntimeInstrumentation()
        .AddAspNetCoreInstrumentation()
        .AddHttpClientInstrumentation()
        .AddPrometheusExporter());
        .AddMeter("YourMeterName"));

Without this, the Prometheus exporter will not be aware of your custom metric.