metrics: New circuit related metrics relay side
This is tied to sponsor 112 with tpo/network-health/team#299 (closed).
We need to implement these:
- Number of circuit collapsing as in DESTROY coming from the opposite direction.
- Circuit being closed due to protocol violation.
- Anything around drop cells. A rate, what type, etc...
Basically, keep counters of those events and expose them on the MetricsPort
.