alternative to #9320, was suggested there

Finally done. The bot is pleased. Sorry for force-pushing this often. I first did not know how to resolve the merge conflict.
Current branch is the original commit merged with 2.8, and then two commits.

What does failure of platformsh mean? Probably nothing about this pr, others fail as well.

Do you want to update profiler.rst to have a link to stopwatch.rst?

Thank you for the PR. Do you think we still need it now that #10259 is merged?

The profiler page would be nice since this is the place I would probably start searching. (Suggested to place the info there in #9320 (review).) But duplicating the information is no help, a link is sufficient.
But dump() is not mentioned on the profiler page. And this is the simplest way to add custom info to the profiler. Should we remain consistent?

summary of history

Well, not sure anymore how history was. I made several PR concerning this topic:
on 2018-02-23 #9320, closed in favour of this one
on 2018-09-05 #10259, merged on 2018-11-30, very similar to #9320, no link to the other two
on 2018-09-12 this one, opened because suggested in #9320 (review)

I guess I created #10259 without remembering the first PR. And having forgotten to mention it when replacing the 1st one. So there were probably two plans...

@SimonHeimberg thanks for this contribution! You are right that this is a bit confusing and it's not easy to know where to put things. So, in #10970 we're revamping the main Profiler article. I've been inspired by your work here to create a section about the Stopwatch integration in the profiler. so, we can close this in favor of #10970. Thanks!