A trace subscriber. It pairs a set of callbacks with the state they need (which can contain a file handle, a socket to write events to, config, etc.).

The design goal for this is that it should be possible to avoid allocations whenever the trace collector invokes the callbacks.

Dummy subscriber that ignores every call.

tee s1 s2 is a subscriber that forwards every call to s1 and s2 both.

Tee multiple subscribers, ie return a subscriber that forwards to all the subscribers in subs.