Exclude very large indexes from reindexing

In order to avoid spikes of activity, we are going to exclude indexes crossing a certain threshold from reindexing.

• Review index size distribution today
• Check production incidents/alerts on weekends - to figure out threshold
• Make sure merge_request_diff_commits, ci_build_trace_sections are being excluded

Alternative solution: Find a way to tag indexes (a COMMENT?) to exclude those from reindexing.