Optimize label sorting

[claytono]

Previously we were sorting labels in every container to build a map key, but also when generating the hash key by calling the LabelsToSignature method. This moves the sorting to be done early in the event processing then passes the sorted label array around.

Before:

$ /usr/local/bin/go test -benchmem -run=^$ github.com/prometheus/statsd_exporter -bench '^(BenchmarkHashNameAndLabels)$' -benchtime 5s
goos: darwin
goarch: amd64
pkg: github.com/prometheus/statsd_exporter
BenchmarkHashNameAndLabels/no_labels-8          300000000               26.7 ns/op             0 B/op          0 allocs/op
BenchmarkHashNameAndLabels/one_label-8          50000000               163 ns/op              48 B/op          2 allocs/op
BenchmarkHashNameAndLabels/many_labels-8        10000000               889 ns/op             192 B/op          2 allocs/op
PASS
ok      github.com/prometheus/statsd_exporter   28.928s

After:

$ /usr/local/bin/go test -benchmem -run=^$ github.com/prometheus/statsd_exporter -bench '^(BenchmarkHashNameAndLabels)$' -benchtime 5s
goos: darwin
goarch: amd64
pkg: github.com/prometheus/statsd_exporter
BenchmarkHashNameAndLabels/no_labels-8          300000000               21.5 ns/op             0 B/op          0 allocs/op
BenchmarkHashNameAndLabels/one_label-8          100000000               59.8 ns/op             0 B/op          0 allocs/op
BenchmarkHashNameAndLabels/many_labels-8        20000000               507 ns/op               0 B/op          0 allocs/op
PASS
ok      github.com/prometheus/statsd_exporter   25.530s

[matthiasr]

Cheers!