Specify individual data instances for the index shard number

[qixian-jiajia]

Description

we know now that we can use the param index.routing.allocation.total_shards_per_node to contol index's shard number for every data instance,I want to have a feature that can define the index number for Specify individual data instances,the reason is that when i add new data instance to the cluster,I can contol the shard number for the new data instance to balance the cluster

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[henningandersen]

@qixian-jiajia I am not sure I understand the request here. Can you elaborate on the purpose of the change you are proposing as well as perhaps provide a concrete example of how you would use this?