fix HashedMetricList when supplying multiple labels

[lukaspj]

Because the order of a map in Go is not deterministic, doing:

m1 := map[string]string {
  "1": "a",
  "2": "b"
}
m2 := map[string]string {
  "1": "a",
  "2": "b"
}

for k, v := range m1 {
  fmt.Println(k, v)
}

for k, v := range m2 {
  fmt.Println(k, v)
}

Does not necessarily yield the same result:

(From Go Playground)