Add support for HDR percentiles in UI

[bcbee]

Describe the feature:

Add the ability to enable HDR mode for Percentiles metrics in the UI. Currently the only way to enable HDR is to add the following to JSON Input:

{"hdr": { "number_of_significant_value_digits" : 3 }}

Describe a specific use case for the feature:

The percentiles aggregation has the ability to use either t-digest or HDR mode to support different workloads (https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-percentile-aggregation.html#_hdr_histogram). HDR is order of magnitudes faster for operations like latency profiling. Providing the ability to easily toggle between the modes would be a huge win for visibility of the feature and ease of use.

[elasticmachine]

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

[timductive]

Closing this because it's not planned to be resolved in the foreseeable future. It will be tracked in our Icebox and will be re-opened if our priorities change. Feel free to re-open if you think it should be melted sooner.