[ML] Model plot terms are not honoured for by-fields

[elasticmachine]

Original comment by @sophiec20:

Found in 6.3.0 "build" : { "hash" : "a47564f", "date" : "2018-04-09T21:45:51.347364Z" },

When a job is configured to use model plot terms, then model plot results should only be written for the specified terms. This is not happening.

farequote job was created for mean(responsetime) by airline with the following model plot:

  "model_plot_config": {
    "enabled": true,
    "terms": "ASA"
  },

Results were written for all airlines

However, when using mean(responsetime) partition airline then only one airline was written, working as expected.

This is related to one of the job validation tests which checks for potentially large cardinality jobs with model plot enabled. LINK REDACTED

[elasticmachine]

Original comment by @dimitris-athanasiou:

I suggest postponing working on this as the rules implementation may change radically.