perf(mu): Implement Push Queue Lag Metrics

[jrdn91]

GIVEN I am a MU operator
WHEN I send a GET to the /metrics endpoint
THEN I should receive OpenMetrics detailing the amount of time each Data Item's "chain" of pushes take to be pushed through the network

The MU worker is in charge of enqueueing and dequeueing messages to process and needs a way to start a timer and stop the timer for a given "top level" message processing, which may resolve with more messages, which any of those messages may resolve with more messages etc.

The current worker logic will have to be retooled in a way to allow for a mechanism to defer a promise of sorts to know when each "level" of outbox messages have processed and weather they contain more messages to process or if the "level" is complete and can then stop the timer.

Relates to #866