Add amount of pending/successful attestations metrics

[KaloyanTanev]

🎯 Problem to be solved

Currently we can track successful attester duties and successful submitted attestations.

However, data would like to know how many attestations we have also submitted (remember, the submission to the BN is a collection of signatures). This is because it's much easier to compare to external sources which track per single attestation.

🛠️ Proposed solution

Add two more metrics:

1. Counter of expected attestations for the slot. Currently data are using pending attester duty, we need something for all attestations expected in the slot.
2. Counter of successful attestations for the slot. Currently data are using successful attester duty, we need something for all attestations successful in the slot. Potentially after successful duty we increase a counter for the amount of attestations?