Expand the log message when CMS.MergeThread completes its merge operation

Expand the log message when CMS.MergeThread completes its merge operation, to include addition useful diagnostic information, like the total-bytes-written, the time taken, as well as rate limiter information.

This issue was filed in response to a question raise on the lucene-dev mailing list, see: https://mail-archives.apache.org/mod_mbox/lucene-dev/202109.mbox/%3cDF40ACBA-6861-48E5-B02F-08E3A05629DF@elastic.co%3e