Expose Group.CopyState() (#4304)

This makes the `rules` package more useful to projects that use
Prometheus as a library.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

rules/manager.go

@@ -277,11 +277,11 @@ func (g *Group) evalTimestamp() time.Time {
 	return time.Unix(0, base+offset)
 }
 
-// copyState copies the alerting rule and staleness related state from the given group.
+// CopyState copies the alerting rule and staleness related state from the given group.
 //
 // 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) {
+func (g *Group) CopyState(from *Group) {
 	g.evaluationDuration = from.evaluationDuration
 
 	ruleMap := make(map[string][]int, len(from.rules))
@@ -498,7 +498,7 @@ func (m *Manager) Update(interval time.Duration, files []string) error {
 		go func(newg *Group) {
 			if ok {
 				oldg.stop()
-				newg.copyState(oldg)
+				newg.CopyState(oldg)
 			}
 			go func() {
 				// Wait with starting evaluation until the rule manager

rules/manager_test.go

@@ -271,7 +271,7 @@ func TestCopyState(t *testing.T) {
 		},
 		seriesInPreviousEval: make([]map[string]labels.Labels, 6),
 	}
-	newGroup.copyState(oldGroup)
+	newGroup.CopyState(oldGroup)
 
 	want := []map[string]labels.Labels{
 		map[string]labels.Labels{"r3a": nil},