Extend the metrics of the Prometheus exporter part 3

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)