[op-dispute-mon] Track the latest proposed L2 block number from games w/ a valid root claim (#11238)

We'd like to know the latest valid block number associated with a valid dispute game.
This would be the equivalent of getting the latest block number from `L2OutputOracle` in pre-FDG.
We could look into the `AnchorStateRegistry`, but this does not give us quick enough feedback
to know if we are keeping up with "proposing" correct root hashes for the most recent L2 blocks,
as dispute games resolves approximately 3.5 days (assuming happy path).

There can be many dispute games at any given time, and they may be valid or invalid.
This metric captures only valid block numbers from dispute games that we "agree" with.