Add histograms to track how long QUIC read/write operations take to complete.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index c4f6eaced25000b3fb4828196b19c157c4d67530..0b62b041d8b3b24629ed662918b46977e873cc4d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -17724,6 +17724,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicSession.AsyncRead">
+  <owner>rch@chromium.org</owner>
+  <summary>
+    True if the result of reading a packet from the network was ERR_IO_PENDING.
+    Recorded for each packet when Read() returns.
+  </summary>
+</histogram>
+
 <histogram name="Net.QuicSession.BlockedFrames.Received">
   <owner>rtenneti@chromium.org</owner>
   <summary>
@@ -18034,6 +18042,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicSession.PacketWriteTime">
+  <owner>rch@chromium.org</owner>
+  <summary>
+    The time taken to Write() a QUIC packet to the socket. Recorded for each
+    packet when it is sent. The suffix specifies if the write completed
+    synchonously or asynchronously.
+  </summary>
+</histogram>
+
 <histogram name="Net.QuicSession.PublicResetAddressMismatch"
     enum="QuicAddressMismatch">
   <owner>wtc@chromium.org</owner>
@@ -58466,6 +58483,12 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="Net.TCP_Connection_Latency_Interval"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="IOMode" separator=".">
+  <suffix name="Synchronous"/>
+  <suffix name="Asynchronous"/>
+  <affected-histogram name="Net.QuicSession.PacketWriteTime"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="IPProtocolType" separator="_">
   <suffix name="UDP"/>
   <suffix name="TCP"/>