[Feature] Add default_granularity to metric spec

[courtneyholcomb]

Is this your first time submitting a feature request?

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

Describe the feature

Add default_granularity to metric spec.
Currently, metric_time defaults to the smallest available granularity. Now that we're enabling sub-daily granularities, users will likely be surprised to see metric_time default to second or anything smaller than day. To enable users to continue defaulting to a more useful granularity while still enabling smaller granularities for a given time dimension, we will allow them to specify their default_granularity in the metric spec. This will default to day, ensuring that this is not a breaking change and the behavior of metric_time does not change until the user specifies a different granularity here.

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