title: Get a registered Spark Accumulator by name
body: <p>Is there a way of getting a registered Spark Accumulator by name, without passing an actual reference? Desired behavior:</p>
  
  <pre><code>val cnt1 = sc.longAccumulator("cnt1")
  val cnt2 = something.getAccumulatorByName("cnt1") asInstanceOf[LongAccumulator]
  cnt1.add(1)
  cnt2.value // returns 1
  </code></pre>
  
  <p>Thanks</p>