Bug: testutil package returning unexpected results

[philipgough]

I'm new to working with Prometheus so apologies if this is a misunderstanding on my part.

I noticed when writing a test for a Counter without any ConstLabels defined in the CounterOpts struct, the test will fail and provide an error message saying two identical strings do not match.

This can be replicated by modifying slightly one of the tests in the testutil_test.go file, by just removing the labels, example:

func TestCollectAndCompareBug(t *testing.T) {
	const metadata = `
		# HELP some_total A value that represents a counter.
		# TYPE some_total counter
	`

	c := prometheus.NewCounter(prometheus.CounterOpts{
		Name: "some_total",
		Help: "A value that represents a counter.",
	})
	c.Inc()

	expected := `

		some_total 1
	`

	if err := CollectAndCompare(c, strings.NewReader(metadata+expected), "some_total"); err != nil {
		t.Errorf("unexpected collecting result:\n%s", err)
	}
}

This results in the following output:

--- FAIL: TestCollectAndCompareBug (0.00s)
    testutil_test.go:165: unexpected collecting result:
        
        metric output does not match expectation; want:
        
        # HELP some_total A value that represents a counter.
        # TYPE some_total counter
        some_total 1
        
        
        got:
        
        # HELP some_total A value that represents a counter.
        # TYPE some_total counter
        some_total 1
        
FAIL

Process finished with exit code 1

I haven't dug in too deeply but if this is a bug and anyone have some pointers to fix I can take it on as soon as I have some free cycles.