Add support for custom DISTRIBUTION metric

[dnlserrano]

Example of usage follows:

iex> MyApp.Statix.distribution("rendering", 12, [])
:ok

From the Datadog documentation:

Unlike the HISTOGRAM metric type, which aggregates on the Agent during a given time interval, a DISTRIBUTION metric sends all the raw data during a time interval to Datadog, and aggregations occur server-side. Because the underlying data structure represents raw, unaggregated data, distributions provide two major features:

• Calculation of percentile aggregations
• Customization of tagging

https://docs.datadoghq.com/developers/metrics/types/?tab=distribution#definition

Closes #46

[dnlserrano]

Can you please review @lexmag? 🙏 Thanks for the library, btw. 😄

[keathley]

@dnlserrano I've incorporated this PR into my fork here: https://github.com/keathley/statix. I'm planning on maintaining that fork going forward.

[dnlserrano]

Great news, thanks @keathley! 🥳

[mtrudel]

Any chance to have this integrated & released?