Add enum for developer resource loads

DevTools front-end CL: https://crrev.com/c/2622195

Bug: chromium:1069378
Change-Id: I51a66ebe85e2036a0939364e0c1e4cb9d525429a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622114
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842946}

chrome/browser/devtools/devtools_ui_bindings.cc

@@ -140,6 +140,8 @@ static const char kDevToolsGridOverlayOpenedFromHistogram[] =
 static const char kDevToolsCssEditorOpenedHistogram[] =
     "DevTools.CssEditorOpened";
 static const char kDevToolsIssueCreatedHistogram[] = "DevTools.IssueCreated";
+static const char kDevToolsDeveloperResourceLoadedHistogram[] =
+    "DevTools.DeveloperResourceLoaded";
 
 static const char kRemotePageActionInspect[] = "inspect";
 static const char kRemotePageActionReload[] = "reload";
@@ -1315,7 +1317,8 @@ void DevToolsUIBindings::RecordEnumeratedHistogram(const std::string& name,
       name == kDevtoolsIssuesPanelResourceOpenedHistogram ||
       name == kDevToolsGridOverlayOpenedFromHistogram ||
       name == kDevToolsCssEditorOpenedHistogram ||
-      name == kDevToolsIssueCreatedHistogram)
+      name == kDevToolsIssueCreatedHistogram ||
+      name == kDevToolsDeveloperResourceLoadedHistogram)
     base::UmaHistogramExactLinear(name, sample, boundary_value);
   else
     frontend_host_->BadMessageReceived();

tools/metrics/histograms/enums.xml

@@ -16883,6 +16883,17 @@ metrics consent we also won't be able to send UMA metrics. -->
   <int value="3" label="Font Editor opened"/>
 </enum>
 
+<enum name="DevToolsDeveloperResourceLoaded">
+  <int value="0" label="LoadThroughPageViaTarget"/>
+  <int value="1" label="LoadThroughPageViaFrame"/>
+  <int value="2" label="LoadThroughPageFailure"/>
+  <int value="3" label="LoadThroughPageFallback"/>
+  <int value="4" label="FallbackAfterFailure"/>
+  <int value="5" label="FallbackPerOverride"/>
+  <int value="6" label="FallbackPerProtocol"/>
+  <int value="7" label="FallbackFailure"/>
+</enum>
+
 <enum name="DevToolsDualScreenDeviceEmulated">
   <int value="0" label="Dual-screen or fold device selected"/>
   <int value="1" label="Dual-screen span mode selected"/>

tools/metrics/histograms/histograms_xml/dev/histograms.xml

@@ -126,6 +126,40 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="DevTools.DeveloperResourceLoaded"
+    enum="DevToolsDeveloperResourceLoaded" expires_after="2021-12-31">
+  <owner>yangguo@chromium.org</owner>
+  <owner>sigurds@chromium.org</owner>
+  <summary>
+    Records for each developer resource load that the DevTools front-end issues
+    (e.g. to load a source map) several enum values encoding which load method
+    was used, whether a fallback was used (and why), and whether the method
+    succeeded or not.
+
+    Some explanations to make sense of the enum values:
+
+    LoadsThroughPage = LoadThroughPageViaTarget + LoadThroughPageViaFrame
+    SuccessfulLoadsThroughPage = LoadsThroughPage - LoadThroughPageFailure
+
+    LoadThroughPageFallback is the number of loads that can in principle be
+    loaded through the page, but had to use the fallback because either loading
+    through the page failed (LoadThroughPageFailure), or there was insufficient
+    information to dispatch the load through the page; the number of such loads
+    can be computed as LoadThroughPageFallback - LoadThroughPageFailure.
+
+    The number of loads not eligible for loading through the page can be
+    computed as FallbackPerOverride + FallbackPerProtocol, where
+    FallbackPerOverride indicates loads where the DevTools user disabled loading
+    through the page via an override, and FallbackPerProtocol indicates loads
+    where DevTools could not load the resource via the page due to the
+    resource's protocol scheme.
+
+    The total number of loads that used the fallback can be computed as
+    LoadThroughPageFallback + FallbackPerOverride + FallbackPerProtocol. Adding
+    SuccessfulLoadsThroughPage to that number gives the total number of loads.
+  </summary>
+</histogram>
+
 <histogram name="DevTools.DualScreenDeviceEmulated"
     enum="DevToolsDualScreenDeviceEmulated" expires_after="M87">
   <owner>yangguo@chromium.org</owner>