[Bug] Make `expr` required for derived metrics

[courtneyholcomb]

Is this a new bug in dbt-semantic-interfaces?

• [X] I believe this is a new bug in dbt-semantic-interfaces
• [X] I have searched the existing issues, and I could not find an existing issue for this bug

Current Behavior

You can define a derived metric without the expr property, but if you try to query it you will get an error.

Expected Behavior

We should have a validation to ensure that the expr property is set for derived metrics.

Steps To Reproduce

Define a derived metric without the expr property. Query it.

Relevant log output

No response

Environment

- OS:
- Python:
- dbt:
- dbt-semantic-interfaces:

Additional Context

No response