Add a UMA histogram to track duration of PrefetchProxy Retry-After's

We expect that this duration will either be pretty short, in the case
of loadshedding origins, or very long in the case of origins trying to
use a 503 response to opt out of prefetches. Since the loadshedding
case is not interesting from a product prespective, this histogram is
scoped out to a week to catch the latter.

Bug: 1146210
Change-Id: Ibeb1488206edbca7c8999a05def7e22d833772a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2538133Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827489}

chrome/browser/prefetch/prefetch_proxy/prefetch_proxy_origin_decider.cc

@@ -7,6 +7,7 @@
 #include <memory>
 #include <vector>
 
+#include "base/metrics/histogram_macros.h"
 #include "base/util/values/values_util.h"
 #include "chrome/browser/prefetch/prefetch_proxy/prefetch_proxy_params.h"
 #include "chrome/browser/profiles/profile.h"
@@ -71,6 +72,10 @@ void PrefetchProxyOriginDecider::ReportOriginRetryAfter(
     return;
   }
 
+  UMA_HISTOGRAM_CUSTOM_TIMES("IsolatedPrerender.Prefetch.Mainframe.RetryAfter",
+                             retry_after, base::TimeDelta::FromSeconds(1),
+                             base::TimeDelta::FromDays(7), 100);
+
   // Cap values at a maximum per experiment.
   if (retry_after > PrefetchProxyMaxRetryAfterDelta()) {
     retry_after = PrefetchProxyMaxRetryAfterDelta();

chrome/browser/prefetch/prefetch_proxy/prefetch_proxy_origin_decider_unittest.cc

@@ -4,6 +4,7 @@
 
 #include "chrome/browser/prefetch/prefetch_proxy/prefetch_proxy_origin_decider.h"
 
+#include "base/test/metrics/histogram_tester.h"
 #include "base/test/scoped_feature_list.h"
 #include "base/test/simple_test_clock.h"
 #include "chrome/browser/prefetch/prefetch_proxy/prefetch_proxy_features.h"
@@ -23,9 +24,12 @@ class PrefetchProxyOriginDeciderTest : public testing::Test {
 
   base::SimpleTestClock* clock() { return &clock_; }
 
+  base::HistogramTester* histogram_tester() { return &histogram_tester_; }
+
  private:
   TestingPrefServiceSimple pref_service_;
   base::SimpleTestClock clock_;
+  base::HistogramTester histogram_tester_;
 };
 
 TEST_F(PrefetchProxyOriginDeciderTest, DefaultEligible) {
@@ -52,6 +56,9 @@ TEST_F(PrefetchProxyOriginDeciderTest, MaxCap) {
 
   decider->ReportOriginRetryAfter(url, base::TimeDelta::FromSeconds(15));
   EXPECT_FALSE(decider->IsOriginOutsideRetryAfterWindow(url));
+  histogram_tester()->ExpectUniqueTimeSample(
+      "IsolatedPrerender.Prefetch.Mainframe.RetryAfter",
+      base::TimeDelta::FromSeconds(15), 1);
 
   clock()->Advance(base::TimeDelta::FromSeconds(11));
   EXPECT_TRUE(decider->IsOriginOutsideRetryAfterWindow(url));
@@ -63,6 +70,9 @@ TEST_F(PrefetchProxyOriginDeciderTest, WaitsForDelta) {
   auto decider = NewDecider();
 
   decider->ReportOriginRetryAfter(url, base::TimeDelta::FromSeconds(15));
+  histogram_tester()->ExpectUniqueTimeSample(
+      "IsolatedPrerender.Prefetch.Mainframe.RetryAfter",
+      base::TimeDelta::FromSeconds(15), 1);
 
   for (size_t i = 0; i <= 15; i++) {
     EXPECT_FALSE(decider->IsOriginOutsideRetryAfterWindow(url));
@@ -77,6 +87,9 @@ TEST_F(PrefetchProxyOriginDeciderTest, ByOrigin) {
 
   decider->ReportOriginRetryAfter(GURL("http://foo.com"),
                                   base::TimeDelta::FromSeconds(1));
+  histogram_tester()->ExpectUniqueTimeSample(
+      "IsolatedPrerender.Prefetch.Mainframe.RetryAfter",
+      base::TimeDelta::FromSeconds(1), 1);
 
   // Any url for the origin should be ineligible.
   for (const GURL& url : {
@@ -105,6 +118,9 @@ TEST_F(PrefetchProxyOriginDeciderTest, Clear) {
   auto decider = NewDecider();
 
   decider->ReportOriginRetryAfter(url, base::TimeDelta::FromSeconds(1));
+  histogram_tester()->ExpectUniqueTimeSample(
+      "IsolatedPrerender.Prefetch.Mainframe.RetryAfter",
+      base::TimeDelta::FromSeconds(1), 1);
   EXPECT_FALSE(decider->IsOriginOutsideRetryAfterWindow(url));
 
   decider->OnBrowsingDataCleared();
@@ -119,6 +135,14 @@ TEST_F(PrefetchProxyOriginDeciderTest, PersistentPrefs) {
                                     base::TimeDelta::FromSeconds(1));
     decider->ReportOriginRetryAfter(GURL("http://foo.com"),
                                     base::TimeDelta::FromSeconds(3));
+    histogram_tester()->ExpectTimeBucketCount(
+        "IsolatedPrerender.Prefetch.Mainframe.RetryAfter",
+        base::TimeDelta::FromSeconds(1), 1);
+    histogram_tester()->ExpectTimeBucketCount(
+        "IsolatedPrerender.Prefetch.Mainframe.RetryAfter",
+        base::TimeDelta::FromSeconds(3), 1);
+    histogram_tester()->ExpectTotalCount(
+        "IsolatedPrerender.Prefetch.Mainframe.RetryAfter", 2);
   }
 
   clock()->Advance(base::TimeDelta::FromSeconds(2));

tools/metrics/histograms/histograms_xml/others/histograms.xml

@@ -7163,6 +7163,17 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="IsolatedPrerender.Prefetch.Mainframe.RetryAfter" units="ms"
+    expires_after="M90">
+  <owner>robertogden@chromium.org</owner>
+  <owner>ryansturm@chromium.org</owner>
+  <owner>tbansal@chromium.org</owner>
+  <summary>
+    Records the duration of a Retry-After header on a 503 response to a
+    mainframe prefetch request.
+  </summary>
+</histogram>
+
 <histogram name="IsolatedPrerender.Prefetch.Mainframe.TotalRedirects"
     units="count" expires_after="M90">
   <owner>robertogden@chromium.org</owner>