Initial POC for metric aggregations

[CrAzE124]

Still a WIP - submitting for some guidance in terms of

• module layout (aggregations.metrics)?
• completeness (should each agg be feature complete - i.e. including "scripts" and related concepts?)

If this looks good I'll move on to the other metric aggs! Looking forward to assisting on this.

[mbuhot]

Looks pretty good to me 👍

For the package name, I’d prefer we either

1. use mbuhot.eskotlin.aggregation to match the existing aggregation builders

Or 2) move the filter / nested aggregation builders into a mbuhot.eskotlin.aggregations.bucket package .

@nzcoffeem what do you think?

As for completeness, feel free to implement as much functionality as you require for your own projects and document any limitations in the code.

Thanks!