提取KV RDD中的所有key生成新的RDD
函数原型：
def keys: RDD[K]
例子：
val a = sc.parallelize(List("dog", "tiger", "lion", "cat", "panther", "eagle"), 2) val b = a.map(x => (x.length, x)) b.keys.collect res2: Array[Int] = Array(3, 5, 4, 3, 7, 5)