Add a way to record multiple observations to a histogram, all having the same value, in a single method call

[vpranckaitis]

Currently, histogram metric interface only allows to record a single observation at a time, using Observe(v float64) method. However, in some cases it might be useful to register multiple identical observations at once, using a single method call, e.g. ObserveMany(v float64, n int). While you could achieve the same thing by running Observe() in a loop, that wouldn't scale for bigger values of n.

Concrete example of a case where this would be useful: on each query, I would like to record its time range duration to a histogram. However, I would also like to weight those observations by the number of items returned in the query response. I could potentially do that with ObserveMany(timeRangeDuration, numberOfItems), but alas, current API doesn't allow to do that.

If this seems like a feature that could be accepted, I could create a PR for it.