Bug 1960923 - Make MathML metric a use counter. r=fredw

That gives us consistent numbers to compare against.

Differential Revision: https://phabricator.services.mozilla.com/D245744

dom/base/Document.cpp

@@ -1360,7 +1360,6 @@ Document::Document(const char* aContentType)
       mRenderingSuppressedForViewTransitions(false),
       mBidiEnabled(false),
       mMayNeedFontPrefsUpdate(true),
-      mMathMLEnabled(false),
       mIsInitialDocumentInWindow(false),
       mIsEverInitialDocumentInWindow(false),
       mIgnoreDocGroupMismatches(false),
@@ -2451,13 +2450,6 @@ Document::~Document() {
 
   if (IsTopLevelContentDocument()) {
     RemoveToplevelLoadingDocument(this);
-
-    // don't report for about: pages
-    if (!IsAboutPage()) {
-      if (MOZ_UNLIKELY(mMathMLEnabled)) {
-        glean::mathml::doc_count.Add(1);
-      }
-    }
   }
 
   mInDestructor = true;

dom/base/Document.h

@@ -1000,8 +1000,6 @@ class Document : public nsINode,
    */
   void SetBidiEnabled() { mBidiEnabled = true; }
 
-  void SetMathMLEnabled() { mMathMLEnabled = true; }
-
   /**
    * Ask this document whether it's the initial document in its window.
    */
@@ -4750,8 +4748,6 @@ class Document : public nsINode,
   bool mBidiEnabled : 1;
   // True if we may need to recompute the language prefs for this document.
   bool mMayNeedFontPrefsUpdate : 1;
-  // True if a MathML element has ever been owned by this document.
-  bool mMathMLEnabled : 1;
 
   // True if this document is the initial document for a window.  This should
   // basically be true only for documents that exist in newly-opened windows or

dom/base/UseCounters.conf

@@ -459,6 +459,8 @@ custom TextDirectivePages used text fragments
 custom InvalidTextDirectives had a text fragment that could not be found after loading has finished
 custom TextDirectiveNotCreated has not succeeded in creating a text fragment
 
+custom MathMLUsed has used MathML
+
 // NOTE: You might need a data collection review if you are adding
 // a custom use counter. And for WebIDL methods or attributes,
 // deprecated operations, or CSS properties: you almost certainly

dom/base/use_counter_metrics.yaml

@@ -107,8 +107,8 @@ use.counter:
     send_in_pings:
       - use-counters
 
-# Total of 2389 use counter metrics (excludes denominators).
-# Total of 354 'page' use counters.
+# Total of 2391 use counter metrics (excludes denominators).
+# Total of 355 'page' use counters.
 use.counter.page:
   svgsvgelement_getelementbyid:
     type: counter
@@ -6128,7 +6128,24 @@ use.counter.page:
     send_in_pings:
       - use-counters
 
-# Total of 354 'document' use counters.
+  math_mlused:
+    type: counter
+    description: >
+      Whether a page has used MathML.
+      Compare against `use.counter.top_level_content_documents_destroyed`
+      to calculate the rate.
+    bugs:
+      - https://bugzilla.mozilla.org/show_bug.cgi?id=1852098
+    data_reviews:
+      - https://bugzilla.mozilla.org/show_bug.cgi?id=1852098
+    notification_emails:
+      - dom-core@mozilla.com
+      - emilio@mozilla.com
+    expires: never
+    send_in_pings:
+      - use-counters
+
+# Total of 355 'document' use counters.
 use.counter.doc:
   svgsvgelement_getelementbyid:
     type: counter
@@ -12148,6 +12165,23 @@ use.counter.doc:
     send_in_pings:
       - use-counters
 
+  math_mlused:
+    type: counter
+    description: >
+      Whether a document has used MathML.
+      Compare against `use.counter.content_documents_destroyed`
+      to calculate the rate.
+    bugs:
+      - https://bugzilla.mozilla.org/show_bug.cgi?id=1852098
+    data_reviews:
+      - https://bugzilla.mozilla.org/show_bug.cgi?id=1852098
+    notification_emails:
+      - dom-core@mozilla.com
+      - emilio@mozilla.com
+    expires: never
+    send_in_pings:
+      - use-counters
+
 # Total of 53 'dedicated worker' use counters.
 use.counter.worker.dedicated:
   pushmanager_subscribe:

dom/mathml/MathMLElement.cpp

@@ -67,7 +67,7 @@ nsresult MathMLElement::BindToTree(BindContext& aContext, nsINode& aParent) {
 
   // Set the bit in the document for telemetry.
   if (Document* doc = aContext.GetComposedDoc()) {
-    doc->SetMathMLEnabled();
+    doc->SetUseCounter(eUseCounter_custom_MathMLUsed);
   }
 
   return rv;

dom/metrics.yaml

@@ -755,28 +755,6 @@ security:
         type: string
     telemetry_mirror: Security_Fissionprincipals_Contentparent
 
-mathml:
-  doc_count:
-    type: counter
-    description: >
-      The number of documents that contained enabled MathML elements.
-      This metric was generated to correspond to the Legacy Telemetry
-      scalar mathml.doc_count.
-    bugs:
-      - https://bugzil.la/1362187
-      - https://bugzil.la/1538985
-      - https://bugzil.la/1590350
-    data_reviews:
-      - https://bugzil.la/1362187
-      - https://bugzil.la/1538985
-      - https://bugzil.la/1590350
-    notification_emails:
-      - emilio@mozilla.com
-      - fred.wang@free.fr
-      - dev-tech-layout@lists.mozilla.org
-    expires: never
-    telemetry_mirror: MATHML_DOC_COUNT
-
 dom.contentprocess:
   build_id_mismatch:
     type: counter