getBloomFilterMask

Returns a 64-bit bloom filter mask for this metadata key, usable by backend implementations to efficiently determine uniqueness of keys (e.g. for deduplication and grouping). This value is calculated on the assumption that there are normally not more than 10 distinct metadata keys being processed at any time. If more distinct keys need to be processed using this Bloom Filter mask, it will result in a higher than optimal false-positive rate.