Add drain method to AtomicHistogram

[rcoh]

Problem

I would like to use AtomicHistogram in a metrics-collection/publishing context. In order to avoid additional synchronization primitives, I need the ability to simultaneously produce a new histogram for analysis while draining all stored values from the histogram.

Some of the docs were also out of date, and I fixed those as well.

Solution

Add a drain method that uses AtomicU64::swap to reset buckets to zero as they are consumed. This method is only available on platforms where 64 bit atomics are available and is cfg-gated accordingly.

[brayniac]

Thanks for opening the PR! This looks like a good addition. I appreciate the doc updates too.

[brayniac]

Included in release 0.10.1 which is now live on crates.io

Thanks again for the contribution!