Allow `/_xpack/usage` endpoint to optionally run on the local node

[legrego]

Currently, /_xpack/usage returns results from the current master node. It would be useful to add a ?local=true option to allow this endpoint to return results from the local node.

Concrete use case: We use this endpoint to detect if external identity realms are enabled, and show a warning in the Role Mappings UI if they aren't. Being able to get results from the local node would give Kibana a more accurate answer when deciding whether or not to show this warning.

Thanks @jasontedor for this suggestion

[bytebilly]

Another use case that mentioned this feature request: Kibana uses the _xpack/usage endpoint to verify it the connection is encrypted, and so allow alerting.

There are probably workarounds for that, and I'm interested in better understanding how this could impact to other scenarions.

How much effort is needed to fetch the status from the local node when a flag is provided, as suggested in the first comment?

[bytebilly]

@rjernst should this be labeled Core/Infra team? 🤔

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[rjernst]

I'm not sure exactly where it belongs, but I've marked it with Core/Infra for now. At glance I don't think this should be too difficult, we already have the local flag for other APIs that normally hit the master.