Merge branch 'js/trace2-cap-max-output-files'

The trace2 output, when sending them to files in a designated
directory, can populate the directory with too many files; a
mechanism is introduced to set the maximum number of files and
discard further logs when the maximum is reached.

* js/trace2-cap-max-output-files:
  trace2: write discard message to sentinel files
  trace2: discard new traces if target directory has too many files
  docs: clarify trace2 version invariants
  docs: mention trace2 target-dir mode in git-config