Track and visualize cluster traffic by index set and index

## What?

Cluster traffic should be tracked on an `index set + index` basis, with the ability to focus on specific items in the UI, to help customers/users more easily identify the contributors to a given daily cluster traffic count.

## Why?

Currently there's a disconnect between the numbers shown when viewing daily cluster traffic and elsewhere. Even assuming a single index set and a daily rotation, the size of the index and the daily cluster traffic count don't match. This alone can be confusing, and mixing in non-daily index rotations (e.g. size-based) only add to the complexity.

While it's possible to use widgets to help identify the distribution for live data, that won't help for indexes that no longer exist. Additionally, customers might appreciate more insight into what contributes to the magic number that counts towards their license.

## Your Environment

* Graylog Version: 5.1.3