cmd/alertmanager: add alertmanager_cluster_enabled metric (#1973)
cmd/alertmanager: add alertmanager_cluster_enabled metric (#1973)

Signed-off-by: Simon Pasquier <38e2d22e48be18381a59028f88390cad04670376@redhat.com>
prometheus/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager