config: Fix overflow checking in global config (#2783)

config/config.go

@@ -380,7 +380,7 @@ func (c *GlobalConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	if err := unmarshal((*plain)(gc)); err != nil {
 		return err
 	}
-	if err := checkOverflow(c.XXX, "global config"); err != nil {
+	if err := checkOverflow(gc.XXX, "global config"); err != nil {
 		return err
 	}
 	// First set the correct scrape interval, then check that the timeout

config/config_test.go

@@ -639,6 +639,9 @@ var expectedErrors = []struct {
 	}, {
 		filename: "target_label_hashmod_missing.bad.yml",
 		errMsg:   "relabel configuration for hashmod action requires 'target_label' value",
+	}, {
+		filename: "unknown_global_attr.bad.yml",
+		errMsg:   "unknown fields in global config: nonexistent_field",
 	},
 }
 

config/testdata/unknown_global_attr.bad.yml

@@ -0,0 +1,2 @@
+global:
+  nonexistent_field: test