[Feature] Add `default_grain` to metric spec

[courtneyholcomb]

Is this your first time submitting a feature request?

• [X] I have read the expectations for open source contributors
• [X] I have searched the existing issues, and I could not find an existing issue for this feature
• [X] I am requesting a straightforward change to existing dbt-semantic-interfaces functionality, rather than a Big Idea better suited to a discussion

Describe the feature

Now that we are enabling sub-daily granularities, the current behavior of metric_time will likely be unexpected to users. If they change any of their time dimensions to use a grain smaller than DAY, metric_time will default to that grain. To avoid this confusion, add a parameter that allows users to choose which grain they want their metric to default to. This will default to DAY if not set to avoid the unexpected behavior change. (If DAY is smaller than the defined granularity, will default to the smallest available granularity.)

Describe alternatives you've considered

No response

Who will this benefit?

No response

Are you interested in contributing this feature?

No response

Anything else?

No response

[Jstein77]

Adding the full metrics spec here. Using cumulative metrics as an example, but default_grain can be defined on any metric.

metrics:
    - name: my_metrics
       type: cumulative
           type_params:
                measure: revenue
       default_grain: day #Optional: defaults to day