docs(prometheus) add missing log for 0.4.1
docs(prometheus) add missing log for 0.4.1

This missing log was removed in this commit : https://github.com/Kong/kong-plugin-prometheus/commit/33c0952707a3160e57c138df2e05e6bb6fc27536#diff-4ac32a78649ca5bdd8e0ba38b7006a1eL13

From #67
Kong/kong,Kong/kong,Kong/kong