[DSL] Introduce new endpoint to expose data stream lifecycle stats

[gmarouli]

We introduce the endpoint GET _lifecycle/stats that exposes stats from the data stream lifecycle run and the data streams and backing indices managed by DSL. For example:

GET _lifecycle/stats?human&pretty
{
  "last_run_duration_in_millis": 2,
  "last_run_duration": "2ms",
  "time_between_starts_in_millis": 9998,
  "time_between_starts": "9.99s",
  "data_streams_count": 2,
  "data_streams": [
    {
      "name": "my-other-stream",
      "backing_indices_in_total": 2,
      "backing_indices_in_error": 1
    },
    {
      "name": "my-data-stream",
      "backing_indices_in_total": 2,
      "backing_indices_in_error": 0
    }
  ]
}

[gmarouli]

@elasticmachine update branch

[gmarouli]

I just moved this helper function in the bottom to be after all the test cases.

[andreidan]

Thanks for this note, this is really helpful when reviewing ❤️

[gmarouli]

@elasticmachine update branch

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[elasticsearchmachine]

Hi @gmarouli, I've created a changelog YAML for you.

[andreidan]

Thanks for adding this Mary, this is really nice ! 🚀

LGTM

[andreidan]

Very nice ! ❤️

[andreidan]

Thanks for this note, this is really helpful when reviewing ❤️

[andreidan]

Chunked response ❤️

[gmarouli]

@elasticmachine update branch