[SPARK-26697][CORE] Log local & remote block sizes.

## What changes were proposed in this pull request?

To help debugging failed or slow tasks, its really useful to know the
size of the blocks getting fetched.  Though that is available at the
debug level, debug logs aren't on in general -- but there is already an
info level log line that this augments a little.

## How was this patch tested?

Ran very basic local-cluster mode app, looked at logs.  Example line:

```
INFO ShuffleBlockFetcherIterator: Getting 2 (194.0 B) non-empty blocks including 1 (97.0 B) local blocks and 1 (97.0 B) remote blocks
```

Full suite via jenkins.

Closes apache#23621 from squito/SPARK-26697.

Authored-by: Imran Rashid <[email protected]>
Signed-off-by: Marcelo Vanzin <[email protected]>

Author: squito

core/src/main/scala/org/apache/spark/storage/ShuffleBlockFetcherIterator.scala

@@ -273,6 +273,8 @@ final class ShuffleBlockFetcherIterator(
     // Split local and remote blocks. Remote blocks are further split into FetchRequests of size
     // at most maxBytesInFlight in order to limit the amount of data in flight.
     val remoteRequests = new ArrayBuffer[FetchRequest]
+    var localBlockBytes = 0L
+    var remoteBlockBytes = 0L
 
     for ((address, blockInfos) <- blocksByAddress) {
       if (address.executorId == blockManager.blockManagerId.executorId) {
@@ -284,13 +286,15 @@ final class ShuffleBlockFetcherIterator(
           case None => // do nothing.
         }
         localBlocks ++= blockInfos.map(_._1)
+        localBlockBytes += blockInfos.map(_._2).sum
         numBlocksToFetch += localBlocks.size
       } else {
         val iterator = blockInfos.iterator
         var curRequestSize = 0L
         var curBlocks = new ArrayBuffer[(BlockId, Long)]
         while (iterator.hasNext) {
           val (blockId, size) = iterator.next()
+          remoteBlockBytes += size
           if (size < 0) {
             throw new BlockException(blockId, "Negative block size " + size)
           } else if (size == 0) {
@@ -317,8 +321,10 @@ final class ShuffleBlockFetcherIterator(
         }
       }
     }
-    logInfo(s"Getting $numBlocksToFetch non-empty blocks including ${localBlocks.size}" +
-        s" local blocks and ${remoteBlocks.size} remote blocks")
+    val totalBytes = localBlockBytes + remoteBlockBytes
+    logInfo(s"Getting $numBlocksToFetch (${Utils.bytesToString(totalBytes)}) non-empty blocks " +
+      s"including ${localBlocks.size} (${Utils.bytesToString(localBlockBytes)}) local blocks and " +
+      s"${remoteBlocks.size} (${Utils.bytesToString(remoteBlockBytes)}) remote blocks")
     remoteRequests
   }