Parse Durations much faster

[bboreham]

Without using a regexp.

This shows up in profiles of Prometheus service discovery, since ParseDuration is called twice per discovered target.

name             old time/op    new time/op    delta
ParseDuration-4    2.54µs ±28%    0.04µs ± 3%   -98.30%  (p=0.008 n=5+5)

name             old alloc/op   new alloc/op   delta
ParseDuration-4      480B ± 0%        0B       -100.00%  (p=0.079 n=4+5)

name             old allocs/op  new allocs/op  delta
ParseDuration-4      2.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)

Code inspired by Go standard library time.ParseDuration, but simplified and supporting different units.

[roidelapluie]

Thanks!

[bboreham]

Now I realise this makes one of the Prometheus tests fail due to changed error message:

--- FAIL: TestPopulateLabels (0.00s)
    manager_test.go:436: 
                Error Trace:    /home/vagrant/src/github.com/prometheus/prometheus/scrape/manager_test.go:436
                Error:          Error message not equal:
                                expected: "error parsing scrape interval: not a valid duration string: \"2notseconds\""
                                actual  : "error parsing scrape interval: unknown unit \"notseconds\" in duration \"2notseconds\""
                Test:           TestPopulateLabels

Do you think it's important to retain consistency of error messages? I.e. shall I fix it here or in the test?