[Transform] add support for percentile aggregation

[hendrikmuhs]

add percentile support to Transform

Related: support for percentile ranks, stats, extended_stats, see separate issues

Because percentile output multiple values the transform will output the result as a nested object while the root name is configurable but the inner names are derived from the configuration. For percentiles with decimal places (99.9), the field will replace the . with an _ to not collide with nested objects. Renaming can be done using a ingest pipeline, e.g. to rename my_percentile.50 to my_median.

example configuration:

"my_percentile": {
        "percentiles": {
          "field": "bytes",
          "percents": [
            10,
            50,
            99.9
          ]
        }
      }

example output:

      "my_percentile" : {
        "99_9" : 9875.0,
        "50" : 5673.5,
        "10" : 604.6000000000001
      }

Values are default mapped to double.

Alternative Histogram

7.6 added a histogram datatype. Storing histograms and calculating percentiles on top of it has various advantages.

Transform should support the histogram agg and write the result into a histogram data type.

This alternative should be implemented in addition, especially for large cases, storing histograms allows updating without full re-processing.

[elasticmachine]

Pinging @elastic/ml-core (:ml/Transform)

[hendrikmuhs]

/CC @pkobziak - you might want to subscribe to this issue