Copy over rule group duration on reload.

This is currently getting lost, this will soon be in a metric and we
don't want it dropping to 0 on every reload.

rules/manager.go

@@ -284,6 +284,8 @@ func (g *Group) offset() time.Duration {
 // Rules are matched based on their name. If there are duplicates, the
 // first is matched with the first, second with the second etc.
 func (g *Group) copyState(from *Group) {
+	g.evaluationTime = from.evaluationTime
+
 	ruleMap := make(map[string][]int, len(from.rules))
 
 	for fi, fromRule := range from.rules {

rules/manager_test.go

@@ -266,6 +266,7 @@ func TestCopyState(t *testing.T) {
 			map[string]labels.Labels{"r3a": nil},
 			map[string]labels.Labels{"r3b": nil},
 		},
+		evaluationTime: time.Second,
 	}
 	oldGroup.rules[0].(*AlertingRule).active[42] = nil
 	newGroup := &Group{
@@ -291,6 +292,7 @@ func TestCopyState(t *testing.T) {
 	}
 	testutil.Equals(t, want, newGroup.seriesInPreviousEval)
 	testutil.Equals(t, oldGroup.rules[0], newGroup.rules[3])
+	testutil.Equals(t, oldGroup.evaluationTime, newGroup.evaluationTime)
 }
 
 func TestUpdate(t *testing.T) {