...and adjust the `totalFailedConnections` metric name
and description.
This commit should make the periodic stats log messages
and the relevant metric look less scary to users:
P2P connection failures are relatively frequent and are usually
not indicative of the proxy operator having done something wrong.
So let's tone the wording down.
See the discussion: https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/merge_requests/516#note_3173677.
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).
This adds a new type of SnowflakeEvent. EventOnProxyStats is triggered
by the periodic task run at SummaryInterval and produces an event with a
proxy stats output string.
