[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.
Can there be a race here, or is this code always called from one thread?