supplying units/metric metadata

I know you can change set metric metadata in the main datadog interface - as explained [here](https://help.datadoghq.com/hc/en-us/articles/204580889-How-can-I-set-up-custom-units-for-custom-metrics-), and there is a way of doing it with an api, but is there any way/plans to expose it in this module?

An optional argument to `histogram` (and friends) would be ideal..you could cache the last metadata values and only send them if there is a change?

Start by reviewing the histogram and related metric-method entry points, then read the linked DataDog documentation on custom metric units and metadata. Done should mean callers can optionally supply metadata for these methods and repeated unchanged metadata is handled according to the issue's proposed caching behavior.

Written by the indexing model from the issue text.