def get_first_column_in_table(table_name, column_metadata):
    """Return a string set with one element: the first column of the table.

    In the case where the first column of each table in a schema is a non-explicitly enforced
    primary key, this function can be used as the primary_key_selector parameter in
    fast_sql_server_reflect to amend Table objects with missing primary keys.

    Args:
        table_name: str, name of the table.
        column_metadata: list of dicts, str -> Any, specifying metadata about the columns in a
                         table. Uses the ORDINAL_POSITION and COLUMN_NAME keys of each dict
                         to determine the first column of the table.

    Returns:
        string set with one element: the first column of the table.
    """
    for column in column_metadata:
        if column["ORDINAL_POSITION"] == 1:
            return {column["COLUMN_NAME"]}

    raise AssertionError(f"Unreachable state reached: {table_name} {column_metadata}")