Instance Administrators should be able to check status of Elasticsearch Index

Problem to solve

Elasticsearch administrators currently have no information about what information has been added to the Index or what the overall status of the Index is. This makes it hard to troubleshoot issues or understand if there are problems with the indexing process.

Intended users

GitLab Instance admins for Elasticsearch

Further details

Proposal

There should be information available to Elasticsearch administrators that informs of the current status of the Index. This should be supported at a high level (XX% Complete) as well as on an individual component level (wiki, project, snippets, etc...).

Permissions and Security

This information should only be available to GitLab instance administrators.

Documentation

Elasticsearch administration documentation should be updated.

Links / references

Old Description

We had a customer attempt to upgrade GitLab and reindex their Elasticsearch database. They had an issue where no search results turned up, but a quick `curl` test seemed to suggest Elasticsearch did have some data.

Right now it's really difficult to get a high-level view of how many projects, issues, merge requests, etc. have been indexed. Is there a way we can add something to the admin page that shows this data? Ideally, there would also a way to search for a term, see the query it sends, and the results it gets back from Elasticsearch.