Adds a test case for https://github.com/dagster-io/dagster/issues/1487
step_metadata_fn is run at execution plan creation time (?) and is used only to support the display of the templated spark submit command in dagit for the spark solids. @natekupp has a plan to rework those solids, and we are tracking removal of this parameter at https://github.com/dagster-io/dagster/issues/1883. metadata is just arbitrary keys you set on the solid.