metrics: use regexp.MatchString for metric name check (#4047)

`(*Regexp).Match` have a string-based equivalent
`(*Regexp).MatchString`. We should use the string version to avoid
unnecessary `[]byte` conversions.

Benchmark:

func BenchmarkMatch(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if match := compileNameRegex.Match([]byte("my_metric_name1")); !match {
			b.Fail()
		}
	}
}

func BenchmarkMatchString(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if match := compileNameRegex.MatchString("my_metric_name1"); !match {
			b.Fail()
		}
	}
}

Result:

goos: linux
goarch: amd64
pkg: go.k6.io/k6/metrics
cpu: AMD Ryzen 7 PRO 4750U with Radeon Graphics
BenchmarkMatch-16          	 1457095	       820.7 ns/op	      16 B/op	       1 allocs/op
BenchmarkMatchString-16    	 1664458	       726.6 ns/op	       0 B/op	       0 allocs/op
PASS
coverage: 0.2% of statements
ok  	go.k6.io/k6/metrics	3.983s

Signed-off-by: Eng Zer Jun <[email protected]>

metrics/registry.go

@@ -35,7 +35,7 @@ const (
 var compileNameRegex = regexp.MustCompile(nameRegexString)
 
 func checkName(name string) bool {
-	return compileNameRegex.Match([]byte(name))
+	return compileNameRegex.MatchString(name)
 }
 
 // NewMetric returns new metric registered to this registry