Off the top of my head, I can think of:
- A binary metric for active or standby
- The size of the pending DN message queues
- A timestamp for when the standby NN last read from shared edit log
- The difference between highest generation stamp seen from the shared edit log and the highest generation stamp seen from any DN
It would probably also be useful to have a DN metric which somehow describes which active/standby NNs its talking to, e.g. "times since last communicated with standby/active NNs."
I'm sure there are others as well. Comments strongly encouraged.