Add document count check option to Reindex action

[untergeek]

For some users, a reindex should be able to verify the before and after document count.

This could be true for a queried reindex (hopefully a clever way to get the document count for such a query could be realized) to verify that the target has the expected document count.

This could be true for a many to one reindex (get doc counts on all source indices, local or remote).

Make a flag to do a source to dest doc count check

[rahst12]

This would be very useful for what I'm doing right now. We're looking at implementing this with a bash script in the meantime. If you (@untergeek) have a workaround, I'd love to see it.

[dudubis]

Short one liner until this gets committed:
curl -s "http://localhost:9200/_cat/indices/$INDEX?v&h=docs.count&pretty" | awk '{s+=$1}END{print s}'
Where INDEX='logstash-2018.11.*'

This will return the sum of docs.count for all daily indices. Later you can run the same command on the new reindexed index and compare the two sums.

[untergeek]

Have you followed #1291? This PR may be merged at some point, but it will have to be optional. If the author of that would change it to be optional, I’d be able to merge it sooner.

[rahst12]

Optional would be great!