ILM policies execution stats

[mokrinsky]

I added some stats for ILM policies execution using _all/_ilm/explain API endpoint.

Example of returning metrics:

elasticsearch_ilm_index_status{action="completed",index="foo_1",phase="completed",step="completed"} 1
elasticsearch_ilm_index_status{action="rollover",index="foo_1",phase="hot",step="check-rollover-ready"} 1
elasticsearch_ilm_index_status{action="shrink",index="foo_1",phase="warm",step="shrunk-shards-allocated"} 1
elasticsearch_ilm_index_status{action="complete",index="foo_1",phase="warm",step="complete"} 1
elasticsearch_ilm_index_status{action="complete",index="foo_1",phase="hot",step="complete"} 1
elasticsearch_ilm_index_status{action="complete",index="foo_1",phase="new",step="complete"} 1
elasticsearch_ilm_index_status{action="",index="foo_1",phase="",step=""} 0

Numeric value represents if index is managed by ILM (I found it unique parameter that could be represented by number in that case), everything other is in tags.

It servers my needs in the way it is, but suggestions are welcome.