feat(metrics): integrate Prometheus metrics into router and main

- /metrics endpoint via promhttp.Handler
- actionHandler: request counter, bytes counter, latency histogram, errors
- Gauge updater: queues/messages count per tenant every 15s
- All metrics labeled by tenant and/or operation

app/cmd/goaws.go

@@ -16,6 +16,7 @@ import (
 	"shared-sqs/app/billing"
 	"shared-sqs/app/conf"
 	"shared-sqs/app/gosqs"
+	"shared-sqs/app/metrics"
 	"shared-sqs/app/models"
 	"shared-sqs/app/persistence"
 	"shared-sqs/app/router"
@@ -136,6 +137,9 @@ func main() {
 	quit := make(chan bool)
 	go gosqs.PeriodicTasks(1*time.Second, quit)
 
+	// Metrics: gauge updater — пересчёт очередей/сообщений per tenant каждые 15 секунд
+	metrics.StartGaugeUpdater(15*time.Second, quit)
+
 	// HTTP сервер с таймаутами
 	srv := &http.Server{
 		Addr:         "0.0.0.0:" + port,