Cache with_label_values more & don't do metrics while holding lock

[Centril]

Description of Changes

Two things happen here that work together to improve performance:

• Calls to with_label_values are cached to the extent possible.
• No metrics are reported while a transaction lock, read-only or mutable, is held.

Changes to traits.rs are:

• Transaction terminal functions (methods consuming transactions, like committing) now also return TxMetrics and sometimes also the reducer name (String) from the ExecutionContext.
• TxData now exposes which tables were affected.

Closes #1099 which was the first attempt at not reporting while holding the lock. The PR has since bitrotted.

API and ABI breaking changes

None

Expected complexity level and risk

3, most of the complication is where datastore transactionality is touched.

Testing

Should be covered by existing tests.