Add support for Histogram Metrics

[justanshuman]

Currently, the namespace ziggurat.metrics does not expose any function to report a histogram metrics which we need.

https://github.com/gojek/ziggurat/blob/master/src/ziggurat/metrics.clj

This has a report-time function which should ideally have been report-histogram function.
Will be sending a PR for the same.

[theanirudhvyas]

@justanshuman you have raised the PR against master. That corresponds to the 3.0 release. I assume you'll want the feature in 2.x release as well. I'll raise a similar PR for that.

[theanirudhvyas]

This has been released in v3.1.0 of Ziggurat