Extend the metrics of the Prometheus exporter part 3

[limemloh]

This issue is tracking potential metrics to add to the built-in Prometheus exporter. The metrics should only be relevant for someone maintaining a node.

Continuation of #710 and #755.

• [ ] Current number of pending transactions. (Transactions not included in a block yet).
• [ ] Total purged transactions.
• [ ] Total dead transactions.
• [ ] Baker equity stake. (Stake of the baker without the delegators).
• [ ] Histogram with gRPC response size. (Will probably require replacing tonic::transport::Server)
• [ ] Count of current gRPC open connections. (Will probably require replacing the tonic::transport::Server)
• [ ] Total invalid/valid network messages received. (Leads to soft ban, making it similar to tracking soft bans).
• [ ] Total count of blocks containing finalization records with the signature of the node (Useful to ensure a finalizer is signing stuff in time)