Finer grained control for throttle events

Throttles now support a throttle_key option that groups events by interpolated values (e.g., user ID) instead of processing them as one queue. When configured, events_per_run applies per unique key, preventing one type of incoming event from slowing down other kinds.

For example, with events_per_run: 2 and throttle_key: << upstream.user_id >>, each user gets 2 events processed per run rather than two event total per run.