Remove Cardano-specific `TxMeasure` dimensions from abstract Consensus layer

#1191 introduced an ad-hoc type class to the abstract Consensus layer
https://github.com/IntersectMBO/ouroboros-consensus/blob/447ae7a5c85a32d0cafa2b3325292a6e678e7f7e/ouroboros-consensus/src/ouroboros-consensus/Ouroboros/Consensus/Ledger/SupportsMempool.hs#L432-L436
that is required by `LedgerSupportsMempool blk`. It is used only for the LocalTxMonitor server implementation.

The goal of this ticket is to remove this Cardano-specific notion from the abstract Consensus layer again, eg by changing the class to sth like
```haskell
class TxMeasureMetrics msr where
txMeasureMetricTxSizeBytes :: msr -> Map MeasureName Natural
```

Start in Ouroboros/Consensus/Ledger/SupportsMempool.hs at lines 432-436 and trace the LedgerSupportsMempool constraint into the LocalTxMonitor server implementation. Identify how TxMeasure dimensions enter the abstract Consensus layer, then remove the Cardano-specific notion while preserving the metrics required by LocalTxMonitor.

Written by the indexing model from the issue text.