Add aggregator backfill, pulling fine+coarse buckets from collectors

cmd/aggregator/main.go

@@ -38,13 +38,6 @@ func main() {
 	merger := NewMerger()
 	cache := NewCache(merger, *source)
 	registry := NewTargetRegistry(collectorAddrs)
-	go cache.Run(ctx)
-
-	for _, addr := range collectorAddrs {
-		sub := NewCollectorSub(addr, merger, registry)
-		go sub.Run(ctx)
-		log.Printf("aggregator: subscribing to collector %s", addr)
-	}
 
 	lis, err := net.Listen("tcp", *listen)
 	if err != nil {
@@ -60,6 +53,17 @@ func main() {
 		}
 	}()
 
+	go cache.Run(ctx)
+
+	for _, addr := range collectorAddrs {
+		sub := NewCollectorSub(addr, merger, registry)
+		go sub.Run(ctx)
+		log.Printf("aggregator: subscribing to collector %s", addr)
+	}
+
+	log.Printf("aggregator: backfilling from %d collector(s)", len(collectorAddrs))
+	go Backfill(ctx, collectorAddrs, cache)
+
 	<-ctx.Done()
 	log.Printf("aggregator: shutting down")
 	grpcServer.GracefulStop()