Mutates the info_n dictionary.

def add_metadata(self, observation_n, info_n, available_at=None):
        """Mutates the info_n dictionary."""
        if self.instance_n is None:
            return

        with pyprofile.push('vnc_env.diagnostics.Diagnostics.add_metadata'):
            async = self.pool.imap_unordered(
                self._add_metadata_i,
                zip(self.instance_n, observation_n, info_n, [available_at] * len(observation_n)))
            list(async)