prom_metrics_test.go

package main

import (
    "bytes"
    "context"
    "io"
    "net/http"
    "testing"

    "github.com/prometheus/client_golang/prometheus/testutil/promlint"
    "github.com/stretchr/testify/require"
)

func TestMetrics(t *testing.T) {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()

    // Set up a metrics server running on `addr`.
    //
    // This function should be the same one main() uses.  For my unit tests, I do a little extra 
    // work here and then invoke main() directly, meaning this test is also a smoke test of main().
    // Putting that extra machinery here would be a distraction, so let's pretend this f'n exists.
    addr, err := startMetricsServer(ctx, ":0")  // `:0` means "use any available port"
    require.NoError(t, err)

    // Connect to the metrics server and get the metrics.
    req, err := http.Get("http://"+addr+"/metrics")
    require.NoError(t, err)
    defer req.Body.Close()
    metricBytes, err := io.ReadAll(req.Body)
    require.NoError(t, err)

    // Create a new linter and lint the metrics.
    metricsLinter := promlint.New(bytes.NewBuffer(metricBytes))
    problems, err := metricsLinter.Lint()
    require.NoError(t, err)

    // If there are any problems, print them out and fail the test.
    for _, problem := range problems {
        t.Error("Prometheus metric problem:", problem)
    }
}