Add the cardano_node_block_txs_int counter to the node

This counter is a best-effort count of how many txs this node has _adopted_ (via ChainSel) since it started running.

- It's OK to ignore rollbacks. And so this counter can just be monotonic. (Someday, if the interest justified it, we could add another monotonic counter that counts how many txs have been rolledback.)
- It's OK to double-count a block, if we end up adopting that block more than once (ie we switch away from it and then we switch back to it).
- That's what's meant by "best-effort" here.
- However, to clarify: if we're validating a candidate in ChainSel and end up _not_ adopting it, then the counter should not be increased by the txs on that candidate.

No existing tracer event contains the necessary information for this counter. But it doesn't seem too onerous to add. Somewhere in the LedgerDB functions called by ChainSel. (And Leios's EBs shouldn't make it tremendously more difficult to maintain this counter.)