Lee Hinman bf1a60130d
[7.x] Add telemetery for data streams (#59433) (#59454)
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
  }
  ...
```
2020-07-13 14:30:11 -06:00
..

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