Add relay_backend explores from looker-hub

These explores are now present in https://github.com/mozilla/looker-hub/tree/main/relay_backend since https://github.com/mozilla/bigquery-etl/pull/6358

relay_backend/relay_backend.model.lkml

@@ -1,7 +1,7 @@
 connection: "telemetry"
 label: "Firefox Relay Backend"
 # Include files from looker-hub or spoke-default below. For example:
-# include: "//looker-hub/relay_backend/explores/*"
+include: "//looker-hub/relay_backend/explores/*"
 # include: "//looker-hub/relay_backend/dashboards/*"
 # include: "views/*"
 # include: "explores/*"