Optimize BuildFQName function

[jkroepke]

The BuildFQName function is used on many exporters multiple times.

The overall benefit might be quite low, but the I'm happy to contribute an optimization.

strings.Builder is available since go 1.10 and should me the current requirements.

% benchstat BenchmarkBuildFQName1.txt BenchmarkBuildFQName2.txt                                             
goos: darwin
goarch: amd64
pkg: github.com/prometheus/client_golang/prometheus
cpu: Intel(R) Core(TM) i7-4870HQ CPU @ 2.50GHz
               │ BenchmarkBuildFQName1.txt │      BenchmarkBuildFQName2.txt      │
               │          sec/op           │   sec/op     vs base                │
BuildFQName1-8                 60.43n ± 0%   45.05n ± 0%  -25.44% (p=0.000 n=20)
BuildFQName2-8                 2.310n ± 1%   2.333n ± 1%   +1.00% (p=0.038 n=20)
BuildFQName3-8                 71.14n ± 1%   50.13n ± 0%  -29.53% (p=0.000 n=20)
geomean                        21.49n        17.40n       -19.04%

Go test commands:

go test -bench="BenchmarkBuildFQName" -run=^BenchmarkBuildFQName -count=20 | tee BenchmarkBuildFQName1.txt

Benchmark File:

func BenchmarkBuildFQName1(b *testing.B) {
	for i := 0; i < b.N; i++ {
		prometheus.BuildFQName("node", "", "usage_seconds_total")
	}
}

func BenchmarkBuildFQName2(b *testing.B) {
	for i := 0; i < b.N; i++ {
		prometheus.BuildFQName("node", "cpu", "")
	}
}

func BenchmarkBuildFQName3(b *testing.B) {
	for i := 0; i < b.N; i++ {
		prometheus.BuildFQName("node", "cpu", "usage_seconds_total")
	}
}

[bwplotka]

Beautiful, thanks!