Help users control index lifecycle policy shard allocation during hot phase with index templates

[cjcenizal]

Users can configure an index template to ensure newly created indices are always allocated on hot nodes (example below). See https://github.com/elastic/elasticsearch/issues/46302 for more context. We can ease the UX in both ILM and in a to-be-scoped settings editor (similar to our mappings editor).

PUT _template/ilm-default
{
  "settings":
  {
    "index.routing.allocation.require.tag":"hot"
  }
}

ILM

In the Index Lifecycle Policy form, we can surface a hint in the UI that suggests users use index templates if they want to control shard allocation during the hot phase.

Settings editor

Once we begin work on the settings editor we should also consider surfacing a hint about how setting "index.routing.allocation.require.tag": "hot" can be used to control shard allocation during an index lifecycle policy's hot phase.

Note: This might be tricky because this hint will only apply when the settings editor is used within the Index Template wizard. It wouldn't make sense in the context of, say, creating or cloning an index.

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)

[cjcenizal]

Related to https://github.com/elastic/kibana/issues/59462.

[elasticmachine]

Pinging @elastic/kibana-management (Team:Kibana Management)