add temp waffle flag default metric (#24284)

The argument flag_undefined_default is soon to be retired
once ARCHBOM-132 is closed. The following will be used to
help ensure the rollout is complete.

- Add a temporary metric if flag_undefined_default is used.
- Add deprecation warning for flag_undefined_default.
- Add minor fix for waffle flag metric when no request found.

ARCHBOM-132