Support multiple metric aggs and multi-value metric aggs #1597
Description
Aggregations such as percentiles return multiple numeric values. You could imagine a situation in which you would want to see these all charted together. For example, chart the 25th, 50th and 75th percentile for home price over the course of a year.
In addition, if we have the ability to chart multi-value metric aggs, then we have the ability to add several metric aggs, eg cardinality AND average, or even different metrics for different fields (eg #374)