[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.
I know there were elegance reasons for why we initially rejected an approach that registers a bytesReadFn with the task (metrics) itself, but now that I'm aware that certain operations interleave reads in this way, it seems like that might be a better option?