[Lens] Filters for each number metric (part of filter ratio)

[wylieconlon]

Because filter ratio is a large effort to implement, we need to break it up into incremental steps. The first step that I can identify is the concept of adding a filter to a specific metric, not to the overall visualization. This would let us separate the concept of "filter ratio" into "ratio of any numbers" and "filters." By having two separate concepts which can be combined, Lens becomes a tool that has an easy learning curve, with advanced features built on the simple features.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[wylieconlon]

The most likely way that we would implement this is using a KQL filter input in the metric. Quickly put this together to illustrate: