cmd/frontend: add quota middleware in record-only mode
cmd/frontend: add quota middleware in record-only mode

Add the Quota middleware to the chain, but configure it only
to collect metrics about how many requests it would have blocked;
do not actually block anything.

Change-Id: I6ed6fbc1688fd35211e090507004cc5efb2fc90d
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/583655
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite