Snowplow: Track adding new metric

What does this MR do?

New metrics can be added using the Add Metric button in the monitoring dashboard. This MR uses Snowplow to track such a user action.

This MR only updates part of the requirements mentioned in #36955 (closed) as the other needs to be verified by backend folks. In order to speed up, I'm breaking it down to smaller MRs

Screenshots

This is how the action looks like and has been verified with Dov here #36955 (comment 250931803)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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