Use `metricKey` instead of `metricId`

What does this MR do?

As part of #204796 (closed) I found that metric database IDs are still required in some cases in the UI, specifically to create new alerts. It doesn't make sense to have the database ID in our metrics view model called anything that is not metricId and to keep 2 so-called ids in the same data structure would be very confusing.

This change switches all occurrences to metricId for metricKey so in a follow up MR, the original metric_id can added back to it can be used in alerts.

• metricKey - to be used in the frontend for unique identification
• metricId - (in follow up MR) to be used in frontend-backend integrations and API calls.

Screenshots

N/A

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Related to #204796 (closed)