doc(discovery/kubernetes): container_is_init meta label

Signed-off-by: sh0rez <me@shorez.de>

docs/configuration/configuration.md

@@ -757,6 +757,7 @@ Available meta labels:
 * `__meta_kubernetes_pod_container_port_name`: Name of the container port.
 * `__meta_kubernetes_pod_container_port_number`: Number of the container port.
 * `__meta_kubernetes_pod_container_port_protocol`: Protocol of the container port.
+* `__meta_kubernetes_pod_container_port_is_init`: Designates whether the container is an [InitContainer](https://kubernetes.io/docs/concepts/workloads/pods/init-containers/)
 * `__meta_kubernetes_pod_ready`: Set to `true` or `false` for the pod's ready state.
 * `__meta_kubernetes_pod_phase`: Set to `Pending`, `Running`, `Succeeded`, `Failed` or `Unknown`
   in the [lifecycle](https://kubernetes.io/docs/concepts/workloads/pods/pod-lifecycle/#pod-phase).