It adds notification related prometheus metrics, including:
- total count for events/success/failure/error
- total count for notification per each status code
- gauge of the pending notification queue
Signed-off-by: tifayuki <tifayuki@gmail.com>
Since the notifications package is now decoupled from storage, we are moving it
to the root package.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
