Include number of containers that have updates available in metrics in monitor mode

Feature Request Checklist

• I checked existing issues and discussions for duplicates.
• This is a feature request, not a support question.

Problem Description

I'm exclusively using Watchtower in monitor mode and have a HomeAssistant Dashbard that shows me if an update is available on each device and the number of containers that need to be updated. In the original Watchtower version when using monitor only mode the metric watchtower_containers_updated was showing the number of updates available, but since I switched to your fork it's always zero.

Desired Solution

Add new metric watchtower_updates_available that shows the number of containers that have an update available regardless of whether automatic updates are enabled or not.

Current Alternatives or Workarounds

Manually checking notifications every week.

Additional Context

No response