Metrics: head_updated_slot_offset_seconds

[kasey]

🚀 Feature Request

Description

An operator has expressed interest in a metric that can be used to track exactly when a head block becomes available for attestation, relative to the start of the slot.

Describe the solution you'd like

The metrics name in the issue title head_updated_slot_offset_seconds is a suggestion, the implementer should feel free to improve on it. But for this metric I suggest we follow prometheus naming best practices and use seconds as the unit suffix, representing milliseconds as a floating point fractional second. Since this measurement should reflect the point at which a validator could successfully make a request for an attestation to sign, we should presumably back track from the relevant validator/API code to determine what exact state change in the node to represent with this metric.

Describe alternatives you've considered

We have metrics tracking processing time, and DA wait time, but these are not relative to the start slot, so a new metric seems to be in order if we want to support this.