Check if label value is valid when unmarhsaling external labels from

YAML, add a test to config_tests for valid/invalid external label
value.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

config/config.go

@@ -304,6 +304,9 @@ func (c *GlobalConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
 		if !model.LabelName(l.Name).IsValid() {
 			return fmt.Errorf("%q is not a valid label name", l.Name)
 		}
+		if !model.LabelValue(l.Value).IsValid() {
+			return fmt.Errorf("%q is not a valid label value", l.Value)
+		}
 	}
 
 	// First set the correct scrape interval, then check that the timeout

config/config_test.go

@@ -714,6 +714,9 @@ var expectedErrors = []struct {
 	}, {
 		filename: "labelname2.bad.yml",
 		errMsg:   `"not:allowed" is not a valid label name`,
+	}, {
+		filename: "labelvalue.bad.yml",
+		errMsg:   `"\xff" is not a valid label value`,
 	}, {
 		filename: "regex.bad.yml",
 		errMsg:   "error parsing regexp",

config/testdata/labelvalue.bad.yml

@@ -0,0 +1,3 @@
+global:
+  external_labels:
+    name: !!binary "/w=="