Add backup/restore Prometheus metrics

[zacharya19]

Is your feature request related to a problem? Please describe.
I'm always frustrated when my DragonflyDB is failing to preform snapshot/restore and I have no visibility of it.
For example: When the IAM of the service is broken and DragonflyDB get's access denied.

Describe the solution you'd like
Expose Promethues metrics such as:

snapshot_failure_count
restore_failure_count

Describe alternatives you've considered
The logs has an error message, but using it for alerts is annoying.

Additional context
Add any other context or screenshots about the feature request here.

[zacharya19]

I will add: it can also be a general counter snapshot count (fail and success) in addition to a gauge with fail count.
Might be more useful that way.

[romange]

Does dragonfly proceed if it does not succeed to load the snapshot at the start?

[zacharya19]

It does stay alive, haven't checked if the connection is healthy.
It can also happened after successful boot (e.g. removing the permission to s3 after boot).