Modify the periodic stats output by standalone snowflake proxies to only
include the data transferred during the time interval being logged. This
is an improvement of previous behaviour that logged the total data
transferred by all proxy connections that were closed within the time
interval being logged..
Closes#40302:
https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/issues/40302
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.