Add stat for how often DNS responses are signed (#2716)
Add stat for how often DNS responses are signed (#2716)

I'm interested in seeing both how often DNS responses we see are signed (mainly for CAA, but
also interested in other query types). This change adds a new counter, `Authenticated`, that can
be compared against the `Successes` counter to find the percentage of signed responses we see.
The counter is incremented if the `msg.AuthenticatedData` bit is set by the upstream resolver.
jfrazelle/boulder,letsencrypt/boulder,ibukanov/boulder,jcjones/boulder,lucasallan/boulder,lucasallan/boulder,bmw/boulder,letsencrypt/boulder,jfrazelle/boulder,jsha/boulder,bmw/boulder,jsha/boulder,ibukanov/boulder,jcjones/boulder,jsha/boulder,jcjones/boulder,ibukanov/boulder,lucasallan/boulder,jfrazelle/boulder,letsencrypt/boulder,jcjones/boulder,jfrazelle/boulder,bmw/boulder