dispatch: cache the formatted aggregation group key

Build the group key once from the route key and immutable group labels
when constructing an aggregation group avoiding repeated label sorting,
formatting, and string Sprintf every time it's needed.

Signed-off-by: Guido Trotter <guido@hudson-trading.com>

Author: Guido Trotter

dispatch/dispatch.go

@@ -614,7 +614,7 @@ type aggrGroup struct {
 	opts     *RouteOpts
 	logger   *slog.Logger
 	routeID  string
-	routeKey string
+	groupKey string
 	matchers labels.Matchers
 
 	alerts   *store.Alerts
@@ -667,7 +667,7 @@ func newAggrGroup(
 	ag := &aggrGroup{
 		labels:   labels,
 		routeID:  r.ID(),
-		routeKey: r.Key(),
+		groupKey: fmt.Sprintf("%s:%s", r.Key(), labels),
 		matchers: r.Matchers,
 		opts:     &r.RouteOpts,
 		timeout:  to,
@@ -697,8 +697,10 @@ func (ag *aggrGroup) fingerprint() model.Fingerprint {
 	return ag.labels.Fingerprint()
 }
 
+// GroupKey returns the key built from the route and immutable group labels.
+// Note that multiple aggregation groups can have the same key.
 func (ag *aggrGroup) GroupKey() string {
-	return fmt.Sprintf("%s:%s", ag.routeKey, ag.labels)
+	return ag.groupKey
 }
 
 func (ag *aggrGroup) String() string {