Create metric for next attestation duty slot

[lucassaldanha]

Context

Rocket Pool Grafana dashboard has a panel to show your validator's next attestation duty. This can help users to choose a good time to update their node or do any other maintenance tasks.

It probably does not help much if you are running heaps of validators, but it can be helpful for solo stakers running a hand full of validators.

Description

We can probably use a labelled gauge. Using the validator id/pubkey as a label. We should also consider if we want to have similar metrics for block publishing and sync commitees.

References

In Lighthouse the metric is called vc_attestation_duty_slot.
In Prysm the metric is called validator_next_attestation_slot.