Onion services should include basic intro/rend stats in their heartbeat logs

In the periodic heartbeat messages, if I have one or more onion services configured, I want to hear (a) how many INTRODUCE2 cells I received since the last heartbeat, and (b) how many times circuit_launch_by_extend_info() got called with purpose CIRCUIT_PURPOSE_S_CONNECT_REND since the last heartbeat. These stats would have helped to discover legacy/trac#24894 (moved) and legacy/trac#24895 (moved).

There's probably a good argument for also hearing "(c) how many times we successfully established a joined rendezvous circuit", but that turns out to actually be a bit hard to track, since it looks like there's no explicit "it worked" acknowledgement in our protocol (whoops). We could approximate it with "number of times we sent a RENDEZVOUS1 cell, and the next cell we got in response was not an END cell with reason END_CIRC_REASON_TORPROTOCOL"?