Dropshot exposes no static metrics

[daniel-levin]

As an operator of an HTTP service/application, I need some basic, static telemetry. While the DTrace probes are useful, they are dynamic. I want a constant stream of static metrics amenable to being rotated off-box and aggregated, and wired up to alarms. While consumers of Dropshot can do this themselves, it would be useful and convenient if Dropshot was able to emit metrics.

Using the metrics crate, Dropshot could expose metrics at a negligible cost (picture tracing but for metrics). Users could decide which metrics reporter to use, if any.

In my experience, it's invaluable to have access to these metrics, which are undifferentiated and common to virtually all web services:

1. Error rate, broken down by 4xx/5xx.
2. Errors-per-API.
3. Request latency.

Dropshot could probably emit these for every user. It would be very convenient.