bug: Metric is double-incremented in promhttp.InstrumentRoundTripperCounter

[hairyhenderson]

It looks like v0.13.0 introduced a bug in promhttp.InstrumentRoundTripperCounter (from #1055, it appears).

Instead of replacing the Inc() call with a wrapped call to exemplarAdd(...), the Inc() call was left in:

https://github.com/prometheus/client_golang/pull/1055/files#diff-7871a83741d29b5f29e3c47d1c284f948c258a0c6fc18895f0c2d4204cc46054R76-R81

The impact is that counter metrics are now double-counted when instrumented with promhttp.InstrumentRoundTripperCounter.

The bug can be observed with this test:

func TestBug(t *testing.T) {
	counter := prometheus.NewCounterVec(
		prometheus.CounterOpts{Name: "requests_total", Help: "A request counter"},
		[]string{"method", "code"},
	)

	reg := prometheus.NewRegistry()
	reg.MustRegister(counter)

	rt := promhttp.InstrumentRoundTripperCounter(counter, http.DefaultTransport)
	c := &http.Client{Transport: rt}

	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.WriteHeader(http.StatusOK)
	}))

	req, _ := http.NewRequestWithContext(context.Background(), http.MethodGet, srv.URL, nil)
	resp, _ := c.Do(req)
	defer resp.Body.Close()

	expected := `
		# HELP requests_total A request counter
		# TYPE requests_total counter
		requests_total{code="200",method="get"} 1
	`

	if err := testutil.GatherAndCompare(reg, strings.NewReader(expected),
		"requests_total",
	); err != nil {
		t.Fatal(err)
	}
}

The output is:

[...]
        Diff:
        --- metric output does not match expectation; want
        +++ got:
        @@ -2,3 +2,3 @@
         # TYPE requests_total counter
        -requests_total{code="200",method="get"} 1
        +requests_total{code="200",method="get"} 2

I'll try to issue a fix for this today!

[hairyhenderson]

#1118 will fix this