Add HS DoS defence stats to heartbeat
We should add entries to our heartbeat about the new DoS defences we added to see how helpful and prevalent they are.
- We should mention how many single-hop connections we blocked (legacy/trac#24962 (moved))
- How many times we applied rate-limiting as an introduction point (legacy/trac#15516 (moved)).
(Marking this as easy since the heartbeat module is not too hard to figure out)