Improve Labels.Compare performance w/stringlabels

I was bored on a train and I spent some amount of time trying to scratch
some nanoseconds off the Labels.Compare when running with stringlabels.

I would be ashamed to admit the real amount of time I spent on it.

The worst thing is, I can't really explain why this is performing so
much better, and someone should re-run the benchmarks on their machine
to confirm that it's not something related to general relativity because
the train is moving. I also added some extra real-life benchmark cases
with longer labelsets (these aren't the longest we have in production,
but kubernetes labelsets are fairly common in Prometheus so I thought it
would be nice to have them).

My benchmarks show this diff:

goos: darwin
goarch: arm64
pkg: github.com/prometheus/prometheus/model/labels
                                       │     old     │                 new                 │
                                       │   sec/op    │   sec/op     vs base                │
Labels_Compare/equal                     5.898n ± 0%   5.875n ± 1%   -0.40% (p=0.037 n=10)
Labels_Compare/not_equal                 11.78n ± 2%   11.01n ± 1%   -6.54% (p=0.000 n=10)
Labels_Compare/different_sizes           4.959n ± 1%   4.906n ± 2%   -1.05% (p=0.050 n=10)
Labels_Compare/lots                      21.32n ± 0%   17.54n ± 5%  -17.75% (p=0.000 n=10)
Labels_Compare/real_long_equal           15.06n ± 1%   14.92n ± 0%   -0.93% (p=0.000 n=10)
Labels_Compare/real_long_different_end   25.20n ± 0%   24.43n ± 0%   -3.04% (p=0.000 n=10)
geomean                                  11.86n        11.25n        -5.16%

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

model/labels/labels_stringlabels.go

@@ -363,12 +363,8 @@ func Compare(a, b Labels) int {
 
 	// Now we know that there is some difference before the end of a and b.
 	// Go back through the fields and find which field that difference is in.
-	firstCharDifferent := i
-	for i = 0; ; {
-		size, nextI := decodeSize(a.data, i)
-		if nextI+size > firstCharDifferent {
-			break
-		}
+	firstCharDifferent, i := i, 0
+	for size, nextI := decodeSize(a.data, i); nextI+size <= firstCharDifferent; size, nextI = decodeSize(a.data, i) {
 		i = nextI + size
 	}
 	// Difference is inside this entry.

model/labels/labels_test.go

@@ -532,6 +532,16 @@ var comparisonBenchmarkScenarios = []struct {
 		FromStrings("aaa", "bbb", "ccc", "ddd", "eee", "fff", "ggg", "hhh", "iii", "jjj", "kkk", "lll", "mmm", "nnn", "ooo", "ppp", "qqq", "rrz"),
 		FromStrings("aaa", "bbb", "ccc", "ddd", "eee", "fff", "ggg", "hhh", "iii", "jjj", "kkk", "lll", "mmm", "nnn", "ooo", "ppp", "qqq", "rrr"),
 	},
+	{
+		"real long equal",
+		FromStrings("__name__", "kube_pod_container_status_last_terminated_exitcode", "cluster", "prod-af-north-0", " container", "prometheus", "instance", "kube-state-metrics-0:kube-state-metrics:ksm", "job", "kube-state-metrics/kube-state-metrics", " namespace", "observability-prometheus", "pod", "observability-prometheus-0", "uid", "d3ec90b2-4975-4607-b45d-b9ad64bb417e"),
+		FromStrings("__name__", "kube_pod_container_status_last_terminated_exitcode", "cluster", "prod-af-north-0", " container", "prometheus", "instance", "kube-state-metrics-0:kube-state-metrics:ksm", "job", "kube-state-metrics/kube-state-metrics", " namespace", "observability-prometheus", "pod", "observability-prometheus-0", "uid", "d3ec90b2-4975-4607-b45d-b9ad64bb417e"),
+	},
+	{
+		"real long different end",
+		FromStrings("__name__", "kube_pod_container_status_last_terminated_exitcode", "cluster", "prod-af-north-0", " container", "prometheus", "instance", "kube-state-metrics-0:kube-state-metrics:ksm", "job", "kube-state-metrics/kube-state-metrics", " namespace", "observability-prometheus", "pod", "observability-prometheus-0", "uid", "d3ec90b2-4975-4607-b45d-b9ad64bb417e"),
+		FromStrings("__name__", "kube_pod_container_status_last_terminated_exitcode", "cluster", "prod-af-north-0", " container", "prometheus", "instance", "kube-state-metrics-0:kube-state-metrics:ksm", "job", "kube-state-metrics/kube-state-metrics", " namespace", "observability-prometheus", "pod", "observability-prometheus-0", "uid", "deadbeef-0000-1111-2222-b9ad64bb417e"),
+	},
 }
 
 func BenchmarkLabels_Equals(b *testing.B) {