Add getTotalBufferedBytes() method fo AsyncTransport

Summary:
When BufferCallback::onEgressBuffered() is called, applications usually want to
know how many bytes were buffered so that can take actions (such as avoiding
that connection, failing fast, etc).
This diff adds getTotalPendingBytes() to AsyncTransport to allow applications
to easily query how many bytes were actually buffered.

Reviewed By: spalamarchuk

Differential Revision: D13252677

fbshipit-source-id: 8ec203f6764e00f52d471321afb549376397eb84

mcrouter/lib/carbon/CarbonQueueAppender.h

@@ -51,7 +51,8 @@ class CarbonQueueAppenderStorage {
     assert(nIovsUsed_ < kMaxIovecs);
 
     struct iovec* nextIov = iovs_ + nIovsUsed_;
-    const auto nFilled = buf.fillIov(nextIov, kMaxIovecs - nIovsUsed_);
+    const auto nFilled =
+        buf.fillIov(nextIov, kMaxIovecs - nIovsUsed_).numIovecs;
 
     if (tcpZeroCopyThreshold_ && !applyZeroCopy_ &&
         buf.capacity() >= tcpZeroCopyThreshold_) {
@@ -109,7 +110,8 @@ class CarbonQueueAppenderStorage {
 
   bool setFullBuffer(const folly::IOBuf& buf) {
     struct iovec* nextIov = iovs_ + nIovsUsed_;
-    const auto nFilled = buf.fillIov(nextIov, kMaxIovecs - nIovsUsed_);
+    const auto nFilled =
+        buf.fillIov(nextIov, kMaxIovecs - nIovsUsed_).numIovecs;
 
     if (nFilled > 0) {
       nIovsUsed_ += nFilled;
@@ -215,7 +217,8 @@ class CarbonQueueAppenderStorage {
     struct iovec* nextIov = iovs_ + nIovsUsed_;
     auto bufCopy = buf;
     bufCopy.coalesce();
-    const auto nFilledRetry = bufCopy.fillIov(nextIov, kMaxIovecs - nIovsUsed_);
+    const auto nFilledRetry =
+        bufCopy.fillIov(nextIov, kMaxIovecs - nIovsUsed_).numIovecs;
     assert(nFilledRetry == 1);
     (void)nFilledRetry;
     ++nIovsUsed_;