Exclude user errors from email error metrics

What does this MR do and why?

Our ~"group::certify" error budgets are disproportionately affected by the inclusion of email errors unrelated to our group. (e.g. if a user sends a blank email, that's not something we control).

This MR excludes those errors from having a metric sent to avoid polluting our data with user errors.

Screenshots or screen recordings

No user-facing changes.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #377512 (closed)