This introduces two new types of Emitters:
1. MultiEmitter, which will forward events to other registered Emitters, and
2. RateLimitedEmitter, which will forward events to a wrapped Emitter, subject
to given rate limits.
The methods in the eventchannel package itself act like a multiEmitter, but is
not actually an Emitter. Now we have a DefaultEmitter, and the methods in
eventchannel simply forward calls to the DefaultEmitter.
The unimplemented syscall handler now uses a RateLimetedEmitter that wraps the
DefaultEmitter.
PiperOrigin-RevId: 260612770
5fdb945a0d