[SPARK-4092] [CORE] Fix InputMetrics for coalesce'd Rdds

When calculating the input metrics there was an assumption that one task only reads from one block - this is not true for some operations including coalesce. This patch simply increments the task's input metrics if previous ones existed of the same read method.

A limitation to this patch is that if a task reads from two different blocks of different read methods, one will override the other.
This seems like a reasonable solution to me. @pwendell you originally had objections to adding the callback the inpuitMetrics object. ( https://github.com/sryza/spark/commit/9f1d77cc0eb0f5206ac52621fb85c99cae56f945) Given this new info, are you okay with continuing down this path?