FR - Add pubkey/validator index to missed attestations

[jonastra]

Currently missed_attestations_count and double_missed_attestations_count is a simple integer.

With multiple validators deployed across different systems, it would be very useful to quickly identify which validators are affected.
Especially for alerting, it would be nice to have an array of validator indexes or pubkeys as labels.

Example:
missed_attestations_count{val_index=["val1","val2","val3","val4"]}

Then for alerting you could do something like this:

The following validators missed attestations last epoch: {{ $labels.val_index }}

[aimxhaisse]

Hello,

We've added this feature as experimental for a while but it likely doesn't scale (each combination of slot/epoch will create a dedicated timeseries so in the long run it explodes cardinality).

We plan to instead add new metrics:

• for the last missed attestation slot, as in, the last slot for which a slot was missed
• for the last missed block

It won't be as precise have having the label (), but can give a ballpark as to when those where missed.