Access to bigtable.admin role memebers

        For example:

        .. literalinclude:: snippets.py
            :start-after: [START bigtable_admins_policy]
            :end-before: [END bigtable_admins_policy]

def bigtable_admins(self):
        """Access to bigtable.admin role memebers

        For example:

        .. literalinclude:: snippets.py
            :start-after: [START bigtable_admins_policy]
            :end-before: [END bigtable_admins_policy]
        """
        result = set()
        for member in self._bindings.get(BIGTABLE_ADMIN_ROLE, ()):
            result.add(member)
        return frozenset(result)