compatible for blank tag value

src/common/dataobj/metric.go

@@ -69,7 +69,7 @@ func (m *MetricValue) CheckValidity(now int64) (err error) {
 	}
 
 	if m.Nid == "" && m.Endpoint == "" {
-		err = fmt.Errorf("nid and endpoint should not be empty")
+		err = fmt.Errorf("nid and endpoint should not be both empty")
 		return
 	}
 
@@ -132,12 +132,16 @@ func (m *MetricValue) CheckValidity(now int64) (err error) {
 		delete(m.TagsMap, k)
 		k = filterString(k)
 		v = filterString(v)
-		if len(k) == 0 || len(v) == 0 {
-			err = fmt.Errorf("tag key and value should not be empty key:%s value:%s", k, v)
+		if len(k) == 0 {
+			err = fmt.Errorf("tag key is blank, metric: %s", m.Metric)
 			return
 		}
 
-		m.TagsMap[k] = v
+		if len(v) == 0 {
+			m.TagsMap[k] = "nil"
+		} else {
+			m.TagsMap[k] = v
+		}
 	}
 
 	m.Tags = SortedTags(m.TagsMap)