Method NormsProducer

Returns a DocValuesProducer to read norms from the index.

NOTE: by the time this call returns, it must hold open any files it will need to use; else, those files may be deleted. Additionally, required files may be deleted during the execution of this call before there is a chance to open them. Under these circumstances an should be thrown by the implementation. are expected and will automatically cause a retry of the segment opening logic with the newly revised segments.