Model threadpool metrics as a stream

[mattrjacobs]

See motivations in #943. This builds upon work in #981.

There are 2 types of threadpool metrics:

1. Events - thread executed, thread rejected
2. Utilization - only valid if sampled

We can recover the events from the Command stream in #981 by adding threadpool info to each event. This would allow us to avoid the cost of maintaining separate paths for writing these metrics.
#981 also provides the start of a first-class sample stream, which is a better way to model sampling-based metrics

[mattrjacobs]

Done in #1047