feat(748): Add compile time feature flag for table metrics

[joshua-spacetime]

Closes #748.

This patch adds a temporary feature flag for enabling table metrics.

Table metrics are potentially recorded many times within a transaction. This recording is synchronous and therefore can have a noticable impact on latency.

This new flag will be enabled by default.
With it, table metrics will be recorded; without it, they won't.

Note this flag is temporary.
It will be removed once metrics are recorded async.

Description of Changes

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API and ABI breaking changes

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[Centril]

I saw more with_label_values in:

• database_intance_context_control
• commit_log
• relational_db
• locking_tx_datastore/mod.rs
• wasm_intance_env
• query
• subscription

[joshua-spacetime]

@Centril these were the only ones I saw in the profile, apart from query duration metrics. I didn't want to remove all of them, since @jdetter probably still wants at least some observability when perf testing. These ones will remove the added latency to reducer calls.

I can also put the query duration metrics behind the flag, but perhaps @jdetter should weigh in if he thinks they're still important for perf testing.

[Centril]

I can also put the query duration metrics behind the flag

I'd suggest two gates in that case, one with the ones in this PR and one with the rest.

[jdetter]

I'm watching this one closely, if we can cover as many metrics as possible with the temporary metrics flag that would be awesome. Let me know when this is ready for testing again 👍

[jdetter]

Excellent, this looks great Josh. I'll include this in the benchmark this weekend 👍

Incorporated Centril's suggestions