Update common Prometheus files (#2915)
Update common Prometheus files (#2915)

Signed-off-by: prombot <37eccbe6e937a212bb1e14487f623bcbc2fac774@googlegroups.com>
GoogleCloudPlatform/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager