Use a unique metric tag for each email handler

What does this MR do?

We log metrics when we process an incoming email, using the same event tag, :receive_email. The different handlers can supply different metric parameters. A condition can occur where in the same sidekiq process, if the Prometheus clients gets the same event but with different types of parameters, it will fail with a labels must have the same signature error. This caused the email to never be processed. Nor was an error email generated.

We now enforce unique metric events to be used: :receive_email_service_desk

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team