[MM-64244] Add websocket disconnect reason metric (#31032)

We've recently spent some effort improving websocket reconnection logic. With this commit, I've augmented the websocket reconnect metric to include a disconnect reason. This will help us measure the impact of these changes in production.

webapp/channels/src/components/post_view/post_body_additional_content/__snapshots__/post_body_additional_content.test.tsx.snap

@@ -115,6 +115,7 @@ exports[`PostBodyAdditionalContent with a normal link Should render the plugin c
         "eventCallback": null,
         "firstConnectCallback": null,
         "firstConnectListeners": Set {},
+        "lastErrCode": null,
         "messageListeners": Set {},
         "missedEventCallback": null,
         "missedMessageListeners": Set {},
@@ -175,6 +176,7 @@ exports[`PostBodyAdditionalContent with a normal link Should render the plugin c
         "eventCallback": null,
         "firstConnectCallback": null,
         "firstConnectListeners": Set {},
+        "lastErrCode": null,
         "messageListeners": Set {},
         "missedEventCallback": null,
         "missedMessageListeners": Set {},