Index stats enhancement: creation date and tier_preference

[3kt]

What

This adds two additional fields to the index stats API call response:

• creation_date
• tier_preference

Both information are transparently pushed from the IndexMetadata class, otherwise natively displayed in the index settings.

A stats call against an index would give such a response:

[...]
  "indices": {
    "foobar": {
      "uuid": "H3BofqE4QWqk21PZk314pg",
      "health": "yellow",
      "status": "open",
      "tier_preference": [
        "data_content"
      ],
      "creation_date": 1730912491278,
      "primaries": {
        "docs": {
          "count": 1,
          "deleted": 0,
          "total_size_in_bytes": 5208
        },
[...]

Why

In their current state, the index stats are hard to consume for users, as most exposed stats are increasing counters. Giving contextual information such as when the index was created or what data tier it currently lives in allows users to answer questions like:

• What is my relative usage (query time, storage, index time, etc...) for one data tier compared to another?
• Within a given data tier, what index is the most used?
• What is my relative usage, as function of the index age?