Migration guide: https://github.com/elastic/kibana/blob/master/src/core/MIGRATION_EXAMPLES.md#elasticsearch-client; breaking changes: https://www.elastic.co/guide/en/elasticsearch/client/javascript-api/current/breaking-changes.html

Question regarding the callCluster method provided to the collectors: Should we replace it with { callCluster (with the deprecated flag), esClient (the new one), savedObjects }?

It is a breaking change, but it will also force the other plugins to adapt their logic to the new esClient if needed.

And, by providing savedObjects, collectors will have a "properly-scoped" SO client when retrieving the usage data.