Add metrics to list non-participating miner IDs

[masih]

Add a self-updating list, ideally in grafana that shows the miner IDs that have not participated in F3. To get this information, look at:

• The signers of each finality certificate compared to the power table used by an instance, and
• The distinct list of miner IDs who have published valid GMessage observed by the observer.

Looking at one of these in isolation is not enough. Because:

• As soon as quorum of DECIDE is reached nodes finalize an instance where a participating miner might not get a chance to append its signature to a cert.
• The point of view of observer is temporal: it may not capture every GMessage

Therefore a combination of both seems more appropriate for getting a reliable list.

There is an alternative way of doing this: tapping into interna GPBFT implementation, similar to the way quorum of senders per phase is measured. This might require storing additional information. Needs a closer look.

[BigLep]

Given there isn't urgency on this issue (because we're seeing ~80% QAP participation in nv25 passive testing), I'm bumping this out to https://github.com/filecoin-project/go-f3/milestone/9