feat(proxy): add failed connection count stats

For the summary log and for Prometheus metrics.

Log output example:

In the last 1h0m0s, there were 7 completed successful connections. 2 connections failed. Traffic Relayed ↓ 321 KB (0.10 KB/s), ↑ 123 KB (0.05 KB/s).

I checked this with the WASM version (!513 (closed)). The log output is working alright. I did not test Prometheus.