Extend metric schema to check if internal_events has an events property

What does this MR do and why?

Add constraint to make sure that any metric that has data_source: internal_events also has an events property defined.

This is to avoid inconsistent metric definitions.

It also migrates the only metric that is currently breaking this new rule.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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 #424846 (closed)