Prometheus Http Listener missing tags

[ricsiLT]

Bug Report

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

• OpenTelemetry.Exporter.Prometheus.HttpListener (1.7.0-rc.1)
• OpenTelemetry.Extensions.Hosting (1.7.0)

Runtime version (e.g. net462, net48, netcoreapp3.1, net6.0 etc. You can find this information from the *.csproj file):

• net8

Symptom

Metric tags not present in http listener path.

What is the expected behavior?

Metric tagged with hostname.

What is the actual behavior?

Response from app on localhost:9464/metrics:

# TYPE custom_state gauge
# UNIT custom_state state
# HELP custom_state state of the application
custom_state 0 1702625390749

# EOF

Reproduce

Custom metrics class:

public class CustomMetrics
{
    private State _state;
    private readonly ObservableGauge<int> _stateGauge;

    public CustomMetrics(IMeterFactory meterFactory)
    {
        using var meter = meterFactory.Create("CustomMeter", "1.0.0");

        _stateGauge = meter.CreateObservableGauge<int>(
            "custom_state",
            () => (int)_state,
            "state",
            "state of the application",
            new List<KeyValuePair<string, object?>> { new("hostname", Environment.MachineName) }
        );
    }

    public void ChangeState(State newState) => _state = newState;
}

public enum State
{
    Idle,
    Working
}

Worker:

public class Worker : BackgroundService
{
    private readonly ILogger<Worker> _logger;
    private readonly CustomMetrics _customMetrics;

    public Worker(ILogger<Worker> logger, CustomMetrics customMetrics)
    {
        _logger = logger;
        _customMetrics = customMetrics;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            _logger.LogInformation("Working");
            _customMetrics.ChangeState(State.Working);
            await Task.Delay(1000, stoppingToken);

            _logger.LogInformation("Idling");
            _customMetrics.ChangeState(State.Idle);
            await Task.Delay(1000, stoppingToken);
        }
    }
}

Program.cs:

var builder = Host.CreateApplicationBuilder(args);
builder.Services.AddHostedService<Worker>();

builder.Services
    .AddSingleton<CustomMetrics>()
    .AddOpenTelemetry()
    .WithMetrics(providerBuilder => providerBuilder
        .AddMeter("CustomMeter")
        .AddPrometheusHttpListener()
    );

var host = builder.Build();
host.Run();

We will close this issue if:

• The repro project you share with us is complex. We can't investigate custom projects, so don't point us to such, please.
• If we can not reproduce the behavior you're reporting.

Additional Context

I see some similar issues in repo, just wasn't sure if this is exactly what is covered in them :(

[sprayzcs]

We are also facing a similar issue with the OpenTelemetry.Exporter.OpenTelemetry (Version 1.7.0) Exporter. The tags from an instrument are not exported to the OTEL collector. We have to add the tags to every measurement.

[james-newell-forge]

For anyone who runs across this ticket, I ran into the same bug and Instrument-level metrics appears to be intentionally not supported because there isn't any such concept in the OpenTelemetry Specification. You can see comments about it here

[cijothomas]

Closing as this is expected behavior today. (Not ruling out the possibility of supporting instrument level tags in the future)