Add queue metrics exporter

Changes:
- Added `x/metrics` package
- Added `tools/metrics_exporter` binary

tools/metrics_exporter/main.go

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"net/http"
+
+	"github.com/hibiken/asynq"
+	"github.com/hibiken/asynq/x/metrics"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/collectors"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+// Declare command-line flags.
+// These variables are binded to flags in init().
+var (
+	flagRedisAddr     string
+	flagRedisDB       int
+	flagRedisPassword string
+	flagRedisUsername string
+	flagPort          int
+)
+
+func init() {
+	flag.StringVar(&flagRedisAddr, "redis-addr", "127.0.0.1:6379", "host:port of redis server to connect to")
+	flag.IntVar(&flagRedisDB, "redis-db", 0, "redis DB number to use")
+	flag.StringVar(&flagRedisPassword, "redis-password", "", "password used to connect to redis server")
+	flag.StringVar(&flagRedisUsername, "redis-username", "", "username used to connect to redis server")
+	flag.IntVar(&flagPort, "port", 9876, "port to use for the HTTP server")
+}
+
+func main() {
+	flag.Parse()
+	// Using NewPedanticRegistry here to test the implementation of Collectors and Metrics.
+	reg := prometheus.NewPedanticRegistry()
+
+	inspector := asynq.NewInspector(asynq.RedisClientOpt{
+		Addr:     flagRedisAddr,
+		DB:       flagRedisDB,
+		Password: flagRedisPassword,
+		Username: flagRedisUsername,
+	})
+
+	reg.MustRegister(
+		metrics.NewQueueMetricsCollector(inspector),
+		// Add the standard process and go metrics to the registry
+		collectors.NewProcessCollector(collectors.ProcessCollectorOpts{}),
+		collectors.NewGoCollector(),
+	)
+
+	http.Handle("/metrics", promhttp.HandlerFor(reg, promhttp.HandlerOpts{}))
+	log.Printf("exporter server is listening on port: %d\n", flagPort)
+	log.Fatal(http.ListenAndServe(fmt.Sprintf(":%d", flagPort), nil))
+}