Adds handler to capture external exits of processes, and tests to verify
this is indeed happening.
Problem: I don't know how to silence crash reports in this case. When I
run the tests, I can see the crash reports I added, but I would like to
silence them in tests. ct_helper:ignore() doesn't seem to cut it the way
I use it, which is probably wrong. I see on the console:
Testing ninenines.cowboy.metrics_SUITE: Starting test, 96 test cases
=ERROR REPORT==== 23-Apr-2021::11:22:09 ===
Ranch listener http, connection process <0.266.0>, stream 1 had its
request process <0.275.0> exit with reason external_exit
which we probably want to silence but I'm not sure how. Help?
