code: def downgrade():
      # This migration cannot safely be downgraded; once metric data with the same
      # (key, timestamp, run_uuid, value) are inserted (differing only in their `step`), we cannot
      # revert to a schema where (key, timestamp, run_uuid, value) is the metric primary key.
      pass
  
imports: from alembic import op
  import sqlalchemy
label_desc: Use image node_id 2 for calling a global function with example usage: downgrade() without return types