relay: Don't make DNS timeout trigger an overload

Tor has configure libevent to attempt up to 3 times a DNS query for a maximum of 5 seconds each. Once that 5 seconds has elapsed, it consider the query "Timed Out" but tor only gets a timeout if all 3 attempts have failed.

For example, using Unbound, it has a much higher threshold of timeout. It is well defined in https://www.nlnetlabs.nl/documentation/unbound/info-timeout/ and has some complexity to it. But the gist is that if it times out, it will be much more than 5 seconds.

And so the Tor DNS timeouts are more of a "UX issue" rather than a "network issue". For this reason, we are removing this metric from the overload general signal.

See tpo/network-health/team#139 (closed) for more information.

Fixes #40527 (closed)

Signed-off-by: David Goulet dgoulet@torproject.org