[SL-2194] Conversion metrics cannot render queries with filters unless the element is in the group by

[tlento]

In adding rendering tests for conversion metrics I noticed that a query like the following will raise an exception:

    query_spec = query_parser.parse_and_validate_query(
        metric_names=("visit_buy_conversion_rate",),
        group_by_names=("visit__referrer_id",),
        where_constraint=PydanticWhereFilter(
            where_sql_template=("{{ TimeDimension('metric_time', 'day') }} = '2020-01-01'")
        ),
    )

This fails with the following error:

AssertionError: All parent nodes should have the same set of linkable instances since all values are coalesced.

This is in the dataflow to sql query plan converter. It's possible this is due to removing filter specs from the conversion metric query as detailed in #1199 but as this may not be related I'm opening a separate issue for investigation.

_SL-2194