#15098 add timed stream element for timing streams, as "extra" element

Updated pull request, squashed code.
It also pulls in the MetricsKit into the 2.3.3 branch, this is OK because it's testing only.

This PR includes two methods `timedIntervalBetween` and `timed`. This should be probably merged after Patrik's changes, so I can rebase on top of them (to make the `Implicits` object usage consistend).

Refs https://github.com/akka/akka/issues/15098