[SL-1802] [Bug] join_to_timespine not applied for conversion metrics

[courtneyholcomb]

Is this a new bug in metricflow?

• I believe this is a new bug in metricflow
• I have searched the existing issues, and I could not find an existing issue for this bug

Current Behavior

If a conversion metric specifies that either of it's input measures should join to time spine, that join never occurs.

Expected Behavior

The join to time spine should occur, ensuring that there is a row for each date in the query.

Steps To Reproduce

Add join_to_timespine: true to either measure in a conversion metric's YAML. Query the metric, and you'll see that the output SQL never joins to time spine. Example YAML:

metric:
  name: visit_buy_conversion_rate_7days_fill_nulls_with_0
  description: conversion rate on visits-buys on a 7 day window, filling nulls with 0 for both input measures
  type: conversion
  type_params:
    conversion_type_params:
      base_measure:
        name: visits
        fill_nulls_with: 0
        join_to_timespine: true
      conversion_measure:
        name: buys
        fill_nulls_with: 0
        join_to_timespine: true
      window: 7 days
      entity: user
      calculation: conversion_rate

Relevant log output

No response

Environment

- OS:
- Python:
- dbt:
- metricflow:

Which database are you using?

No response

Additional Context

No response

_SL-1802