Add metrics for `app_name` used in notification email

[matrixbot]

This issue has been migrated from #12992.

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Description:

Spawning from @reivilibre getting a spurious Riot branded email, matrix-org/synapse#11295 (comment)

Could be useful to monitor how much of each app_name we're emailing about. I think we would expect zero (or close to zero) Riot and Vector emails going out. Would be a good signal to see if we're doing something wrong there.