Add median for histogram metrics

[Mr0grog]

When using DogStatsD, the histogram type calculates a <metric_name>.median metric that this package does not. It would be great to add that here.

(Context: I’m experimenting with transitioning an app from DogStatsD to this for an environment where the other system/host metrics DogStatsD collects aren’t needed, so we can reduce configuration overhead and cost.)

I’d be happy to submit a PR to add this, if helpful.

[ErikBoesen]

Sounds good, feel free to make a PR!

[Mr0grog]

Done! #69.