Backports #56745 to 7.x
In the process of developing a new implementation for the Elasticsearch Rollups functionality we came up with the concept of the aggregate metric field type.
The aggregate_metric_double field type can store the results of aggregations (currently min, max, sum, value_count and avg are supported - more to come).
This field allows us to run (min, max, sum, value_count, avg) aggregations on the container field and the field will return the correct metric depending on the aggregation that is computed.
This commit adds data stream info to the `/_xpack` and `/_xpack/usage` APIs. Currently the usage is
pretty minimal, returning only the number of data streams and the number of indices currently
abstracted by a data stream:
```
...
"data_streams" : {
"available" : true,
"enabled" : true,
"data_streams" : 3,
"indices_count" : 17
}
...
```
