diagnostic for 12184: Add a call to channel_dump_statistics

changes/further-12184-diagnostic

@@ -0,0 +1,2 @@
+  o Minor features (diagnostic):
+    - Slightly enhance the diagnostic message for bug 12184.

src/or/circuitbuild.c

@@ -183,6 +183,8 @@ get_unique_circ_id_by_chan(channel_t *chan)
       if (0)
         circuitmux_assert_okay(chan->cmux);
 
+      channel_dump_statistics(chan, LOG_WARN);
+
       return 0;
     }