Divide stats collector in sub-collectors.

In order to avoid timeouts in case of many clients,
it should be possible to configure which stats should
be collected.

Implements the following sub-collectors:
 * topics
 * channels
 * clients

collector/nsqd.go

@@ -0,0 +1,62 @@
+package collector
+
+import (
+	"sync"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// StatsCollector defines an interface for collecting specific stats
+// from a nsqd node.
+type StatsCollector interface {
+	collect(s *stats, out chan<- prometheus.Metric)
+}
+
+// NsqdStats represents a Collector which collects all the configured stats
+// from a nsqd node. Besides the configured stats it will also expose a
+// metric for the total number of existing topics.
+type NsqdStats struct {
+	nsqdURL    string
+	collectors []StatsCollector
+	topicCount prometheus.Gauge
+}
+
+// NewNsqdStats creates a new stats collector which uses the given namespace
+// and reads the stats from the given URL of the nsqd.
+func NewNsqdStats(namespace, nsqdURL string) *NsqdStats {
+	return &NsqdStats{
+		nsqdURL: nsqdURL,
+		topicCount: prometheus.NewGauge(prometheus.GaugeOpts{
+			Namespace: namespace,
+			Name:      "topics_total",
+			Help:      "The total number of topics",
+		}),
+	}
+}
+
+// Use configures a specific stats collector, so the stats could be
+// exposed to the Prometheus system.
+func (s *NsqdStats) Use(c StatsCollector) {
+	s.collectors = append(s.collectors, c)
+}
+
+// Collect collects all the registered stats metrics from the nsqd node.
+func (s *NsqdStats) Collect(out chan<- prometheus.Metric) error {
+	stats, err := getNsqdStats(s.nsqdURL)
+	if err != nil {
+		return err
+	}
+
+	var wg sync.WaitGroup
+	wg.Add(len(s.collectors))
+	for _, coll := range s.collectors {
+		go func(coll StatsCollector) {
+			coll.collect(stats, out)
+			wg.Done()
+		}(coll)
+	}
+
+	s.topicCount.Set(float64(len(stats.Topics)))
+	wg.Wait()
+	return nil
+}