For #15369: Add documentation for synced tabs opening metrics.

app/metrics.yaml

@@ -331,6 +331,21 @@ events:
     notification_emails:
       - fenix-core@mozilla.com
     expires: "2021-04-01"
+  synced_tab_opened:
+    type: event
+    description: >
+      An event that indicates that a synced tab was opened.
+    send_in_pings:
+      - metrics
+    bugs:
+      - https://github.com/mozilla-mobile/fenix/issues/15369
+    data_reviews:
+      - https://github.com/mozilla-mobile/fenix/pull/16727
+    data_sensitivity:
+      - interaction
+    notification_emails:
+      - fenix-core@mozilla.com
+    expires: "2021-05-10"
   recently_closed_tabs_opened:
     type: event
     description: |

docs/metrics.md

@@ -266,6 +266,7 @@ The following metrics are added to the ping:
 | downloads_management.item_deleted |[event](https://mozilla.github.io/glean/book/user/metrics/event.html) |A counter for how often a user deletes one / more downloads at a time.  |[1](https://github.com/mozilla-mobile/fenix/pull/16728)||2021-04-01 | |
 | downloads_management.item_opened |[event](https://mozilla.github.io/glean/book/user/metrics/event.html) |A counter for how often a user tap to opens a download from inside the "Downloads" folder.  |[1](https://github.com/mozilla-mobile/fenix/pull/16728)||2021-04-01 | |
 | downloads_misc.download_added |[event](https://mozilla.github.io/glean/book/user/metrics/event.html) |A counter for how many times something is downloaded in the app. |[1](https://github.com/mozilla-mobile/fenix/pull/16730)||2021-04-01 | |
+| events.synced_tab_opened |[event](https://mozilla.github.io/glean/book/user/metrics/event.html) |An event that indicates that a synced tab was opened.  |[1](https://github.com/mozilla-mobile/fenix/pull/16727)||2021-05-10 |2 |
 | events.total_uri_count |[counter](https://mozilla.github.io/glean/book/user/metrics/counter.html) |A counter of URIs visited by the user in the current session, including page reloads. This does not include background page requests and URIs from embedded pages or private browsing but may be incremented without user interaction by website scripts that programmatically redirect to a new location.  |[1](https://github.com/mozilla-mobile/fenix/pull/1785), [2](https://github.com/mozilla-mobile/fenix/pull/8314), [3](https://github.com/mozilla-mobile/fenix/pull/15713#issuecomment-703972068)||2021-08-01 |2 |
 | metrics.adjust_ad_group |[string](https://mozilla.github.io/glean/book/user/metrics/string.html) |A string containing the Adjust ad group ID from which the user installed Fenix. This will not send on the first session the user runs. If the install is organic, this will be empty.  |[1](https://github.com/mozilla-mobile/fenix/pull/9253), [2](https://github.com/mozilla-mobile/fenix/pull/15713#issuecomment-703972068)||2021-08-01 |2 |
 | metrics.adjust_campaign |[string](https://mozilla.github.io/glean/book/user/metrics/string.html) |A string containing the Adjust campaign ID from which the user installed Fenix. This will not send on the first session the user runs. If the install is organic, this will be empty.  |[1](https://github.com/mozilla-mobile/fenix/pull/5579), [2](https://github.com/mozilla-mobile/fenix/pull/15713#issuecomment-703972068)||2021-08-01 |1 |