Expose index uuid in _stats endpoint

[ruflin]

To collect metrics about the Elasticsearch indices Metricbeat calls the /_stats endpoint. Inside the object "indices" : { all indices are listed with its names. The data that is missing inside is the index uuid. It can be fetched by calling GET /index-name and then can be found under uuid. Having to make a separate request for each index to fetch the identifier is not an optimal solution.

The uuid is important for the following reasons:

• The same index name can exist in multiple clusters. Beats can collect data from multiple clusters into a single monitoring cluster.
• An index can be deleted and a new one created with the same name. In this case only the UUID is different.
• With cross cluster replication matching indices from 2 different clusters with the same uuid could become even more interesting: Introduce cross-cluster replication #30086

NOTE: The _stats endpoint also currently does not expose the cluster_uuid and we fetch it in a separate call.

[elasticmachine]

Pinging @elastic/es-core-infra

[rjernst]

We discussed this in FixItThursday and it sounds reasonable and should be relatively easy to do. I'm marking this as adopt me for anyone to pick up.

[ruflin]

Thanks, great to hear.

[original-brownbear]

Opened #31871 for this one