Hidden indices not exposed in monitoring index_stats documents

[chrisronline]

With the introduction of hidden indices, it looks like they are not included in the monitoring index_stats documents. (It's worth noting that the index total in cluster_stats monitoring documents is accurate (cluster_stats.indices.count)).

I'm also proposing we add additional mappings to indicate that the index is hidden, such as index_stats.hidden: <boolean>.

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Monitoring)

[dakrone]

Internal monitoring has been deprecated for quite a while, and we're no longer doing any active development on it. I'm going to close this issue.