model: sort label names without boxing them into sort.Interface

[SoloJacobs]

SignatureForLabels becomes allocation-free, which matters because it is the only exported way to hash a subset of a label set. Alertmanager hit this in its inhibitor: it calls the equivalent once per inhibition rule per alert on GET /api/v2/alerts.

    benchstat over 8 runs (n=8, p=0.000 throughout):
    
      benchmark                     sec/op              B/op        allocs/op
      SignatureForLabels/1     60.71n -> 17.68n      24 -> 0       1 -> 0
      SignatureForLabels/2     95.40n -> 40.34n      24 -> 0       1 -> 0
      SignatureForLabels/3     121.40n -> 70.86n     24 -> 0       1 -> 0
      SignatureWithoutLabels   214.2n -> 184.8n      72 -> 48      2 -> 1

Benchmark here:

func BenchmarkSignatureForLabels(b *testing.B) {
	m := Metric{
		"first-label":  "first-label-value",
		"second-label": "second-label-value",
		"third-label":  "third-label-value",
	}

	for _, n := range []int{1, 2, 3} {
		labels := make([]LabelName, 0, n)
		for ln := range m {
			labels = append(labels, ln)
			if len(labels) == n {
				break
			}
		}
		slices.Sort(labels)

		b.Run(strconv.Itoa(n)+"-labels", func(b *testing.B) {
			b.ReportAllocs()
			for b.Loop() {
				SignatureForLabels(m, labels...)
			}
		})
	}
}

func BenchmarkSignatureWithoutLabels(b *testing.B) {
	m := Metric{
		"first-label":  "first-label-value",
		"second-label": "second-label-value",
		"third-label":  "third-label-value",
	}
	exclude := map[LabelName]struct{}{"first-label": {}}

	b.ReportAllocs()
	for b.Loop() {
		SignatureWithoutLabels(m, exclude)
	}
}