Log more information when service discovery fails

Adds the current host list length to the logged information when service
discovery fails. This will help to identify times when service discovery
fails and leaves the application with an empty host list.

lib/gitlab/database/load_balancing/service_discovery.rb

@@ -89,8 +89,10 @@ module Gitlab
             # Sentry, instead of silently terminating this thread.
             Gitlab::ErrorTracking.track_exception(error)
 
-            Gitlab::AppLogger.error(
-              "Service discovery encountered an error: #{error.message}"
+            Gitlab::Database::LoadBalancing::Logger.error(
+              event: :service_discovery_failure,
+              message: "Service discovery encountered an error: #{error.message}",
+              host_list_length: load_balancer.host_list.length
             )
 
             # Slightly randomize the retry delay so that, in the case of a total