Make metric changes of master commits conveniently available

In !3318 (comment 280478), Simon notes that it should be more apparent what kind of metric changes a commit incurred.

Just the Metric (In|De)creases section of a commit message isn't enough: I want to see which jobs increased/decreased (I see there is syntax for that) and by how much (no syntax). Attaching this info to the commit message seems brittle, as a rebase easily invalidates all info.

Fortunately, all the information is already available in the form of git notes, but there's no easy way to see that metadata. I imagine some kind of report by Marge posted under the MR, or even better attached to the master commit in some view of GitLab, reading approximately

"compile_time/bytes_allocated" Decreases:
  T1234 [linux-deb9-unreg-hadrian (-2.4%), linux-deb9-dwarf (-42.3%)]
"compile_time/max_bytes_used" Decreases:
  T9834 [Darwin (-12.4%), linux-deb9-dwarf (-3%)]

Or some such.

Or like what we have in the test runner.