discovery/kubernetes: fix selectors validation for role pod

Signed-off-by: Haley Wang <haley@victoriametrics.com>

config/testdata/kubernetes_selectors_pod.good.yml

@@ -7,7 +7,9 @@ scrape_configs:
             label: "foo=bar"
             field: "metadata.status=Running"
       - role: pod
+        attach_metadata:
+          node: true
         selectors:
-          - role: "pod"
-            label: "foo in (bar,baz)"
+          - role: "node"
+            label: "foo=bar"
             field: "metadata.status=Running"

discovery/kubernetes/kubernetes.go

@@ -222,6 +222,10 @@ func (c *SDConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
 				break
 			}
 		}
+		// The pod role supports node selectors when configured with `attach_metadata: {node: true}`.
+		if !allowed && selector.Role == RoleNode && c.Role == RolePod && c.AttachMetadata.Node {
+			allowed = true
+		}
 
 		if !allowed {
 			return fmt.Errorf("%s role supports only %s selectors", c.Role, strings.Join(allowedSelectors[c.Role], ", "))