autoscaler-agent should have billing request duration histogram metrics

[sharnoff]

Problem description / Motivation

Currently it's difficult to see "typical" billing request duration timings. Fixing the issue with lastSendDuration (ref #787) should help somewhat here, but still doesn't show the complete picture.

Feature idea(s) / DoD

A new histogram metric for "send" duration. This could either record the total duration of sending metrics, including when it's split into multiple parts (like lastSendDuration), or the duration of the individual HTTP requests. Or maybe both - but it should be clear either way.

We might also want to have the histogram differentiate between successful vs unsuccessful outcomes, because those two are likely to have significantly different timing distributions.