Go test utility to dump Prometheus metrics observed during the test.

metrics.go

package testutil

import (
	"bytes"
	"flag"
	"os"
	"testing"

	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/common/expfmt"
)

var flagMetrics = flag.Bool("metrics", false, "Enables dumping Prometheus metrics to stdout")

func NewMetricsRegistry(t *testing.T) *prometheus.Registry {
	registry := prometheus.NewRegistry()

	t.Cleanup(func() {
		if !*flagMetrics {
			return
		}

		mfs, err := registry.Gather()
		if err != nil {
			t.Logf("Error gathering Prometheus metrics: %s", err)
			return
		}

		var buf bytes.Buffer
		encoder := expfmt.NewEncoder(&buf, expfmt.FmtText)

		for i, mf := range mfs {
			if err := encoder.Encode(mf); err != nil {
				t.Logf("Error encoding Prometheus metrics family %d: %s", i, err)
				return
			}
		}

		t.Log("Prometheus metrics:")
		os.Stdout.WriteString(buf.String())
	})

	return registry
}

In the test code initialize metrics registry and register collectors you want to observe.

Example output:

$ go test -v -metrics
=== RUN   TestHandler
    metrics.go:39: Prometheus metrics:
# HELP wsnotifier_websocket_connection_duration_seconds 
# TYPE wsnotifier_websocket_connection_duration_seconds summary
wsnotifier_websocket_connection_duration_seconds_sum 9.6651e-05
wsnotifier_websocket_connection_duration_seconds_count 1
# HELP wsnotifier_websocket_handshake_duration 
# TYPE wsnotifier_websocket_handshake_duration summary
wsnotifier_websocket_handshake_duration_sum 9.5392e-05
wsnotifier_websocket_handshake_duration_count 1
# HELP wsnotifier_websocket_open_connection 
# TYPE wsnotifier_websocket_open_connection gauge
wsnotifier_websocket_open_connection 0
--- PASS: TestHandler (0.00s)
PASS
ok      gitlab.com/adc-sistemi/backend/websocket-notifier/ws    0.004s