Better parachain liveness and finality metrics

[sandreim]

To help us further make decision on what are the most impactful areas to invest in for improving parachain throughput and latency we need to make some measurements. Some of the metrics required should already exist so what is needed is just some dashboard, but other stuff needs to be implemented as well.

High level view of what is the current impact on parachains:

• Produced parachain blocks
• Finalized parachain blocks
• Finalized to produced ratio

To further drill down and understand why, we also need more parachain consensus observability.

Prospective parachains candidate drop reasons:

• Parachain produced forks
• Candidate’s relay parent went out of scope before it could get backed on chain ( Availability of a previously backed candidate took more than one block, Session change? Invalidate a complete prospective chain; Relay chain block was full ? Block producer did not put the candidate in for unknown reasons?)

Other metrics of interest:

• Statement distribution: candidate’s relay parent went out of scope before it was successfully backed by enough backers.
• Availability: availability timed out and thus a candidate was dropped.
• Relay chain reorgs
• Collator protocol: the candidate could not be transmitted to backers; throttling (fairness)
• Backing dropped invalid candidates