Move "Didn't recognize cell, but circ stops here" into heartbeat.

When we looked, this was the third most frequent message at
PROTOCOL_WARN, and doesn't actually tell us what to do about it.
Now:
 * we just log it at info
 * we log it only once per circuit
 * we report, in the heartbeat, how many times it happens, how many
   cells it happens with per circuit, and how long these circuits
   have been alive (on average).

Fixes the final part of #40400.

1 files changed, 7 insertions, 0 deletions

diff --git a/changes/bug40400_part3 b/changes/bug40400_part3
new file mode 100644
index 0000000000..b78e800229
--- /dev/null
+++ b/changes/bug40400_part3
@@ -0,0 +1,7 @@
+  o Minor features (logging, heartbeat):
+    - When a relay receives a cell that isn't encrypted properly for
+      it, but the relay is the last hop on the circuit, the relay
+      now counts how many cells of this kind it receives, on how
+      many circuits, and reports this information in the log.
+      Previously, we'd log each cell at PROTOCOL_WARN level, which
+      is far too verbose to be useful. Fixes part of ticket 40400.