email: Use tracing to log errors and successes

[Turbo87]

Using add_custom_metadata() in the email code makes it hard to port the log_request middleware to axum, due to the CUSTOM_METADATA thread local which appears to be hard to migrate into the async world. The easiest solution for now is to use tracing calls directly instead and log the messages outside of the regular HTTP traffic logs.

Related:

• Log sent emails and error out if non-SMTP is used in production #4627