Mock Prometheus Middleware in unit tests

[Jasstkn]

Hi!

I've configured Prometheus Middleware with custom metric as shown in the example.

I've tried to mock echo with this Middleware in the tests but it looks like it doesn't work the way I expect it to work and middleware is being initialised for every test.

func TestGetToken(t *testing.T) {
	for _, tt := range []struct {
		name           string
		client         ClientStub
		expectedStatus int
		expectedBody   string
	}{
		{
			name:           "successful token generation",
			client:         ClientStub{},
			expectedStatus: http.StatusOK,
		},
		{
			name:           "unsuccessful token generation",
			client:         ClientStub{err: errors.New("error")},
			expectedStatus: http.StatusInternalServerError,
		},
	} {
		t.Run(tt.name, func(t *testing.T) {
			// Setup
			e := server.NewEchoServer()
			req := httptest.NewRequest(http.MethodGet, "/token", nil)
			rec := httptest.NewRecorder()
			c := e.NewContext(req, rec)
			logger := zaptest.NewLogger(t)

			// init metrics
			m := metrics.NewMetrics()
			c.Set(metrics.CustomKeyMetrics, m)
			p := prometheus.NewPrometheus(metrics.CustomKeyMetrics, nil, m.MetricList())
			p.Use(e)
			e.Use(m.AddCustomMetricsMiddleware)
			h := GetToken(context.Background(), logger, &tt.client)
			h(c)
			// Assertions
			assert.Equal(t, tt.expectedStatus, rec.Code)

			t.Cleanup(func() {
				e.Close()
			})
		})
	}
}

The test produces following errors in the logs:

{"time":"2023-03-13T21:07:03.703121+01:00","level":"ERROR","prefix":"-","file":"prometheus.go","line":"382","message":"token_count could not be registered in Prometheus: duplicate metrics collector registration attempted"}
{"time":"2023-03-13T21:07:03.703145+01:00","level":"ERROR","prefix":"-","file":"prometheus.go","line":"382","message":"requests_total could not be registered in Prometheus: duplicate metrics collector registration attempted"}
{"time":"2023-03-13T21:07:03.70316+01:00","level":"ERROR","prefix":"-","file":"prometheus.go","line":"382","message":"request_duration_seconds could not be registered in Prometheus: duplicate metrics collector registration attempted"}
{"time":"2023-03-13T21:07:03.703177+01:00","level":"ERROR","prefix":"-","file":"prometheus.go","line":"382","message":"response_size_bytes could not be registered in Prometheus: duplicate metrics collector registration attempted"}
{"time":"2023-03-13T21:07:03.703189+01:00","level":"ERROR","prefix":"-","file":"prometheus.go","line":"382","message":"request_size_bytes could not be registered in Prometheus: duplicate metrics collector registration attempted"}

Therefore it looks like middleware isn't properly isolated but I can't figure out how to do it. Can you advise?

[aldas]

This is because middleware.NewPrometheus() registers metrics into global register here https://github.com/labstack/echo-contrib/blob/b6855c2b7d9745b7db2498623e89cae526b41f57/prometheus/prometheus.go#L381

Prometheus middleware own tests unregister these metrics like that at the end of test https://github.com/labstack/echo-contrib/blob/b6855c2b7d9745b7db2498623e89cae526b41f57/prometheus/prometheus_test.go#L15

p.s. Prometheus middleware repository is located here https://github.com/labstack/echo-contrib and issues/PR should be sent there.

[Jasstkn]

Got it, thanks a lot! In my case unregister func looks like this:

func unregister(p *prometheus.Prometheus) {
	// unregister all metrics
	for _, m := range p.MetricsList {
		prometheus.Unregister(m.MetricCollector)
	}
}