Timer.ObserveDuration should returned the observed duration

[eklitzke]

The following usage example is given in the docs:

func TimeMe() {
    timer := NewTimer(myHistogram)
    defer timer.ObserveDuration()
    // Do actual work.
}

I find that I typically want to also emit durations in log statements. Therefore it would be useful if ObserveDuration() returned the observed duration (probably as a time.Duration, but I could see a case being made for the value as a float64), which would allow code like the following:

func TimeMe() {
    timer := NewTimer(myHistogram)
    defer func() { log.Printf("finished in %f seconds", timer.ObserveDuration().Seconds()) }()
    // Do actual work.
}

[beorn7]

Sounds like a convenient and reasonable thing to do. And easy to implement. :o)