Add chart showing total merge time by part of index

[mikemccand]

Lucene's infoStream now logs how long each part of the index (postings, doc values, points, vectors, etc.) take to merge. It should be simple to parse these logs from nightly benchmarks and chart these merge costs over time?

[mikemccand]

Hmm, except, we do not enable infoStream logging in the nightly benchmarks! We could enable it for the single-threaded index, since we don't otherwise count its metrics like docs/sec.

[rmuir]

yes this would be nice when discussing issues such as apache/lucene#12203
otherwise, I think merges are currently too opaque when discussing index performance: but we "know" certain parts are way slower than others.

honestly as an even simpler start we could simply make the infostream.txt available somehow (if it isn't too insane amount of disk space). then at least we could investigate the current breakdown.