Bug 1538985 - Renew the telemetry for documents that contain MathML, and make it opt-out. r=janerik

There's renewed interest in MathML, see https://github.com/mathml-refresh/.

We want to have data before removing deprecated stuff, from our core MathML
implementation.

We want to measure the release population, and compare number of documents with
MathML against all top-level content documents. The comparison is meaningless if
the data is collected in a different way from the way we collect top-level
content document loads, so given that the scalar for top-level loads is opt-out,
make it match.

Differential Revision: https://phabricator.services.mozilla.com/D24812

--HG--
extra : moz-landing-system : lando

toolkit/components/telemetry/Scalars.yaml

@@ -889,13 +889,16 @@ mathml:
   doc_count:
     bug_numbers:
       - 1362187
+      - 1538985
     description: >
       The number of documents that contained enabled MathML elements.
-    expires: "63"
+    expires: "73"
     kind: uint
     notification_emails:
-      - gsquelart@mozilla.com
-    release_channel_collection: opt-in
+      - emilio@mozilla.com
+      - fredw@igalia.com
+      - dev-tech-layout@lists.mozilla.org
+    release_channel_collection: opt-out
     record_in_processes:
       - 'content'