Add analytics plugin usage stats to _xpack/usage

[imotov]

In 7.6.2 _xpack/usage is returning

  "analytics" : {
    "available" : true,
    "enabled" : true,
    "stats" : [
      {
        "cumulative_cardinality_usage" : 0
      },
      {
        "cumulative_cardinality_usage" : 0
      },
      {
        "cumulative_cardinality_usage" : 0
      },
      {
        "cumulative_cardinality_usage" : 0
      }
    ]
  }

and in master we get:

  "analytics" : {
    "available" : true,
    "enabled" : true
  }

We should add cumulative stats for each aggregation instead.

[elasticmachine]

Pinging @elastic/es-analytics-geo (:Analytics/Aggregations)

[nik9000]

Oops.