Allow customisation of "preferences" for _msearch

[alepuccetti]

Describe the feature:

Kibana bundle all dashboard queries into a single _msearch. There are already few tickets about how to migrate to single searches or multiple smaller _msearch. (e.g. #14750).

However, it would be an improvement if it was possible to configure Kibana in a way that the user could specify if using preferences or leave it out of the requests. As explained in this comment elastic/elasticsearch#27775 (comment), preferences ensures that all search inside a dashboard use the same shards/replicas. So if you have an index with 1 shard and n replica Kibana will not exploit elasticsearch parallelism.

On a side note, I would be curious to know why this was designed like this in the first place.

[alepuccetti]

Any update on this?

[nreese]

there is a PR out for review if you want to mess around with the feature and provide feedback #17271

[alepuccetti]

Awesome, I will give it a go and see what happen. Thank you @nreese