DAL node: add a metric counting unattested slots seen

What

Add a DAL node Prometheus metric that counts unattested slots per slot index.

Why

We want visibility into how many published slots go unattested so operators can monitor DAL attestation coverage.

How

Introduce slots_unattested as a labeled Prometheus Counter in dal_metrics and expose slot_unattested in the interface. Increment it on every Unattested status update in the statuses cache; keep attested/waiting gauges unchanged.

A Counter is used to avoid resets/decrements and to accumulate events over time.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR