Making Metric Timestamps Optional

[callum-mcdata]

resolves #6398

Description

This makes the timestamp property optional. This is in order to support non time-grain behavior in dbt_metrics. PR here: dbt-labs/dbt_metrics#204

Concrete changes:

• Makes timestamp optional
• Raises a ValidationError if time_grains are defined but are missing timestamp
• Raises a ValidationError if window is defined but is missing timestamp
• Adds tests to ensure that all three above pieces of functionality are working as expected

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[jtcohen6]

@callum-mcdata I think this requires an update to docs (for v1.4+), since we currently say that timestamp is required in this table. Does that sound right to you?

[callum-mcdata]

@jtcohen6 yep! I've got it on my to-do - we're going to mirror the readme in dbt_metrics over to the docs page so people don't need to reference two locations.

[callum-mcdata]

@jtcohen6 if you're curious here is the PR for updating the docs

[jtcohen6]

@callum-mcdata You're the best! Was just reviewing this morning to make sure we're not missing any relevant docs updates