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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _rpc_metadata(self):
 """The RPC metadata for this client's associated database. Returns: Sequence[Tuple(str, str)]: RPC metadata with resource prefix for the database associated with this client. """

        if self._rpc_metadata_internal is None:
            self._rpc_metadata_internal = _helpers.metadata_with_prefix(
                self._database_string
            )

        return self._rpc_metadata_internal