allow tag keys to contain underscores

tsdb/store.go

@@ -1500,7 +1500,7 @@ func (s *Store) TagValues(auth query.Authorizer, shardIDs []uint64, cond influxq
 			switch e.Op {
 			case influxql.EQ, influxql.NEQ, influxql.EQREGEX, influxql.NEQREGEX:
 				tag, ok := e.LHS.(*influxql.VarRef)
-				if !ok || strings.HasPrefix(tag.Val, "_") {
+				if !ok || influxql.IsSystemName(tag.Val) {
 					return nil
 				}
 			}