Sending the time of the pingback request in the request

components/data_reduction_proxy/proto/pageload_metrics.proto

Index: components/data_reduction_proxy/proto/pageload_metrics.proto
diff --git a/components/data_reduction_proxy/proto/pageload_metrics.proto b/components/data_reduction_proxy/proto/pageload_metrics.proto
index b6240cfcdd894a80dde55b0bfde98753c2feea0e..aa6500f0977c091d1fb9867502b9943e11b6f26f 100644
--- a/components/data_reduction_proxy/proto/pageload_metrics.proto
+++ b/components/data_reduction_proxy/proto/pageload_metrics.proto
@@ -7,20 +7,22 @@ syntax = "proto2";
 option optimize_for = LITE_RUNTIME;
 
 import "client_config.proto";
 
 package data_reduction_proxy;
 
 // Request message to record metrics for one or more pageloads.
 message RecordPageloadMetricsRequest {
   // The pageload metrics to record.
   repeated PageloadMetrics pageloads = 1;
+  // Time metrics were sent.
+  optional Timestamp metrics_sent_time = 2;
 }
 
 // Metrics for a single pageload.
 message PageloadMetrics {
   // The session key used to load the page.
   optional string session_key = 1;
   // The time at which the first request of the pageload was made, according to
   // the client's clock.
   optional Timestamp first_request_time = 2;
   // The time at which the last request of the pageload was made, according to