Timer helpers

[cl-bvl]

Hello.

I'm proposing to add several helper functions to make working with timers more convenient.
I haven’t brought this up on the mailing list yet, but I’d like to hear your feedback.
The code is still a draft; if you like the approach, I’ll add more detailed comments, tests, and documentation.

Here are a few examples of how the new timers could be used:

// Counter with SUM of all measurements
timerCounter := NewTimerCounter(Opts{Name: "timer_counter"})
go func() {
	defer timerCounter.Observe()()
	time.Sleep(2 * time.Second)
}()

// Histogram with all measurements
timerHistogram := NewTimerHistogram(HistogramOpts{Name: "timer_histogram", Buckets: []float64{1, 2, 3, 4, 5, 6}})
timerHistogram.Wrap(func() {
	time.Sleep(time.Duration(rand.Intn(5000)) * time.Millisecond)
})

// Vector histogram. Most other types of timers also have a vector version.
timerHistogramVec := NewTimerHistogramVec(HistogramOpts{Name: "timer_histogram_vec", Buckets: []float64{1, 2, 3, 4, 5, 6}}, []string{"name"})
go func() {
	defer timerHistogramVec.Observe(map[string]string{"name": "timer1"})()
	time.Sleep(time.Duration(rand.Intn(5000)) * time.Millisecond)
}()
timerHistogramVec.WrapLabelValues([]string{"timer2"}, func() {
	time.Sleep(time.Duration(rand.Intn(5000)) * time.Millisecond)
})

// Counter with SUM of all measurements
// But the value also shows the sum of running timers. 
// It is well suited for long-running tasks - then the metric shows their running time until their completion.
timerContinuous := NewTimerContinuous(Opts{Name: "timer_continuous"}, 0)
timerContinuous.Wrap(func() {
	time.Sleep(5 * time.Minute)
})