Metric to track Postgres errors

[matthughes]

I'd love a counter metric that tracked every Postgres error Skunk raises. I could take a stab at a PR if there is interest and you think this fits the project's goals.

[iRevive]

https://opentelemetry.io/docs/specs/semconv/database/database-metrics/ - there are official database semantic conventions, but they are still in the 'experimental' / 'development' state.
But they should be marked stable, presumably this year.

We can follow the convention even for the draft implementation.

[iRevive]

There is an experimental generation of instruments from semantic conventions: typelevel/otel4s#764.

If we merge it, we can use it to test the implementation.

[satabin]

FYI: I wanted to take a stab at this one, using semconv for traces and adding metrics, I'll try to open a PR soon.