index.number_of_replicas behavior with index.auto_expand_replicas

[pickypg]

When using index.auto_expand_replicas with a value other than false (its default), index.number_of_replicas is silently ignored:

DELETE /test

PUT /test
{
  "settings": {
    "number_of_shards": 1,
    "number_of_replicas": 2,
    "auto_expand_replicas": "0-1"
  }
}

# reports "number_of_replicas" as 1 (on a multi-node cluster)
GET /test/_settings

# Shows the cluster as green with test having 1 replica
GET /_cat/indices/test?v

# Responds with { "acknowledged": true }
PUT /test/_settings
{
  "number_of_replicas": 0
}

# "test" index still has 1 replica
GET /_cat/indices/test?v

# reports number_of_replicas as 1
GET /test/_settings

# request properly succeeds
PUT /test/_settings
{
  "auto_expand_replicas": false
}

# still reports "number_of_replicas" as 1
GET /test/_settings

# actually changes "number_of_replicas" to 0
PUT /test/_settings
{
  "number_of_replicas": 0
}

I kind of wonder if we should just deprecate auto_expand_replicas and add its functionality to number_of_replicas instead?

[pickypg]

/cc @spinscale

[spinscale]

@pickypg duplicate of #27835 ?

[pickypg]

Indeed! I didn't notice it when I created this one.