statistics: Properly count all rendezvous cells (avoid undercounting). [035+]
tl;dr We were not counting cells flying from the client to the service, but we were counting cells flying from the service to the client.
When a rendezvous cell arrives from the client to the RP, the RP forwards it to the service.
For this to happen, the cell first passes through command_process_relay_cell()
which normally does the statistics counting. However because the rend_circ
circuit was not flagged with circuit_carries_hs_traffic_stats in
rend_mid_rendezvous(), the cell is not counted there.
Then the cell goes to circuit_receive_relay_cell() which has a special code
block based on rend_splice specifically for rendezvous cells, and the cell
gets directly passed to rend_circ via a direct call to
circuit_receive_relay_cell(). The cell never passes through
command_process_relay_cell() ever again and hence is never counted by our
rephist module.
The fix here is to flag the rend_circ circuit with
circuit_carries_hs_traffic_stats so that the cell is counted as soon as it
hits command_process_relay_cell().
Furthermore we avoid double-counting cells since the special code block of circuit_receive_relay_cell() makes us count rendezvous cells only as they enter the RP and not as they exit it.
Fixes #40117 (closed).