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.

In particular:

• 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)