Monitoring: Add monitoring watch to display disabled allocation in Kibana

[spinscale]

A common pitfall when doing a rolling upgrade is to disable shard allocation as outlined in our docs, but then forget to reenable it again when the upgrade is done.

We could add another watch to monitoring, that actually is checking for this.

This would require changes in the ClusterStatsMonitoringDoc, which already contains the cluster state, so one can extract state.metaData().transientSettings() and state.metaData().persistentSettings() and check for the allocation one, and then send this field over as well.

The next step would be add a watch that could make use of that field, so that an error message gets displayed in Kibana.

[elasticmachine]

Pinging @elastic/es-core-infra