This feature is part of elastic/kibana#61738

When Kibana runs BackgroundSessions, it will need to have a monitoring service that periodically checks for the status of each session. We will use that information to mark sessions that are complete or have errors. It will also be responsible for sending push notifications back to users.

In order to do that, we need to have access to each _async_search's metadata, using the kibana admin user (rather than the user that created the search). This meta data should include the amount of total shards, amount of shards processed and the is_running and is_partial flags.