static <K, V> ImmutableMap<K, V> singleton(K key, V value) {
        return PCollectionsImmutableMap.singleton(key, value);
    }
@param key the key
@param value the value
@return a wrapped hash-based persistent map that has a single mapping
@param <K> the key type
@param <V> the value type
server-common/src/main/java/org/apache/kafka/server/immutable/ImmutableMap.java