metrics: use regexp.MatchString for metric name check

[Juneezee]

What?

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

Why?

It's a one-line change that provides a free performance gain.

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

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

[ankur22]

Thanks! LGTM 🚀