nsqd: record delivery to FIN metrics

[mreiferson]

We currently have end-to-end (from PUB to FIN) metrics in NSQ. We need another distinct set of "e2e" metrics that measure delivery (the point in time nsqd wrote the message to the wire) to FIN.

This way you could separate the overall end-to-end processing health from the current health of the consumption side.

Obviously, when the queue is 30mm behind the existing E2E metrics are gonna go through the roof (and from a business perspective it's a good metric to know).

But, for debugging, you want to be able to understand how fast your workers are currently able to process messages. This would help identify when it's a consumer/downstream problem vs. something else. i.e. If it normally takes 2seconds to process messages but now it's taking 10seconds, that would help explain why you have a backlog.

cc @elubow

[allgeek]

Just FYI I'm taking a crack at this and hope to submit a PR, but am partially blocked by uncertainty regarding the timing of changes for #323... don't want to submit a stale PR with changes to the old nsqadmin.