Prototype how to use collect status in Prometheus

[sed-i]

If we're going to have constant [log/status message] strings this way, they could be class-level rather than instance level, or just plain ALL-CAPS constants the way the old status was (just to make it clear what they are).

Alternatively, this seems like a great place for a tiny meta-config class/method, where:

if some_bad_thing:
  self.set_unit_status("push_fail") # or even an Enum at the top to make it self.set_unit_status(PUSH_FAIL)

Would be a nice abstraction which keeps the list of conditions->statuses in a single place, almost like a excepthook

Originally posted by @rbarry82 in #352 (comment)

[simskij]

This is meant to be addressed by the collect status feature of canonical/operator#954