Refactors our use of `slogtest` to instantiate a "standard logger" across most of our tests. This standard logger incorporates https://github.com/coder/slog/pull/217 to also ignore database query canceled errors by default, which are a source of low-severity flakes.
Any test that has set non-default `slogtest.Options` is left alone. In particular, `coderdtest` defaults to ignoring all errors. We might consider revisiting that decision now that we have better tools to target the really common flaky Error logs on shutdown.
Before db_metrics were all or nothing. Now `InTx` metrics are always recorded, and query metrics are opt in.
Adds instrumentation & logging around serialization failures in the database.
