Create protocol abstraction for Measures

[QMalcolm]

We want a standard definition of what a valid Measure implementation should have. This allows for a shared definition to be understood in MetricFlow and dbt-core of what anything implementing the Measure protocol will make available without MetricFlow and dbt-core needing to import each other. This should use the new Protocol type which exists in python 3.8+

[callum-mcdata]

Proposed implementation

class NonAdditiveDimensionParametersProtocol(Protocol):
    name: str
    window_choice: AggregationType
    window_groupings: List[str]

class MeasureAggregationParametersProtocol(Protocol):
    percentile: Optional[float]
    use_discrete_percentile: bool
    use_approximate_percentile: bool

class MeasureProtocol(Protocol):
    name: str
    agg: AggregationType
    description: Optional[str]
    create_metric: Optional[bool]
    expr: Optional[str]
    agg_params: Optional[MeasureAggregationParametersProtocol]
    non_additive_dimension: Optional[NonAdditiveDimensionParametersProtocol]
    agg_time_dimension: Optional[str]
    checked_agg_time_dimension: TimeDimensionReference
    reference: MeasureReference

[callum-mcdata]

@WilliamDee can I get your 👀 on this to see if it passes the sniff test?

[WilliamDee]

@WilliamDee can I get your 👀 on this to see if it passes the sniff test?

@callum-mcdata This looks right! Question I had is do we necessarily need the properties being a protocol as well (ie., MeasureAggregationParametersProtocol)? The answer to that would probably be how flexible we want to be with building these custom classes.

[callum-mcdata]

I think @QMalcolm is probably the best person to answer this. I have no strong opinons one way or another on how we want to implement this 🤔

Additionally I've edited the proposal based on a conversation he and I had offline around whether we want default implementations for property methods. He recommended that we instead define the shape of those properties and core is free to implement them as they see fit.