Also add OTP-20.1.4 to CI.

Another experimental stream handler. It enables tracing for
the connection process and any children processes based on
the matching of the request. It can be used to do ad-hoc
tracing by sending a specific header, path, method or other.

It is meant to be used both for tests and production. Some
configuration scenarios are NOT safe for production, beware.

It's important to understand that, at this time, tracing
is enabled on the scale of the entire connection including
any future request processes. Keep this in mind when trying
to use it in production. The only way to stop tracing is
by having the callback function exit (by calling exit/1
explicitly). This can be done after a certain number of
events for example. Tracing can generate a lot of events,
so it's a good idea to stop after a small number of events
(between 1000 and 10000 should be good) and to avoid tracing
the whole world.

Documentation will follow at a later time.

It collects metrics and passes them to a configurable callback
once the stream terminates. It will be documented in a future
release. More tests incoming.

Hello 2.0.0!

This should work very similar to normal supervisors,
in particular during the shutdown sequence when the
connection process goes down or switches to Websocket.

Processes that need to enforce the shutdown timeout
will be required to trap exits, just like in a supervisor.
In a vanilla Cowboy, this only matters at connection
shutdown, as Cowboy will otherwise wait for the request
process to be down before stopping the stream.

Tests are currently missing.