Support field datatype rather than is_time

[jonmmease]

I'd suggest that a logical datatype field be used in place of the proposed is_time field. For consumers of semantic models, the data type can be important when building logic on top of dimensions. A motivating example is a BI tool that supports its own expression language on top of dimensions to compute local derived fields or local measures for visualization. Having correct type information can be critical for working with dimensions properly.

I would suggest enumeration values of:

• number
• string
• boolean
• time
• timestamp
• timestamptz
• opaque/other (for anything else)

For reference: LookML, Cube, and Hex require this information in their specs

[sfc-gh-agoldberg]

This shouldn't be limited to fields - having datatype on metrics would have the same value (especially if the metric is defined with an unknown dialect)

[khush-bhatia]

I created an issue from this discussion for tracking. If anyone's interested in taking it up, here is the direct link
#84