Add simple anonymous usage metrics (#1167)

This change reports total function call count to an analytics url once a day.

It's done behind the existing analytics flag.

router/analytics.go

@@ -0,0 +1,61 @@
+package router
+
+import (
+	"bytes"
+	"encoding/json"
+	"net/http"
+	"os"
+	"sync/atomic"
+	"time"
+
+	"github.com/dchest/uniuri"
+)
+
+type (
+	Analytics struct {
+		id  string
+		url string
+	}
+	AnalyticsData struct {
+		id                string
+		FunctionCallCount uint64 `json:"FunctionCallCount"`
+	}
+)
+
+func MakeAnalytics(url string) *Analytics {
+
+	if len(url) == 0 {
+		url = os.Getenv("ANALYTICS_URL")
+		if len(url) == 0 {
+			return nil
+		}
+	}
+
+	a := &Analytics{
+		url: url,
+		id:  uniuri.NewLen(8),
+	}
+	go a.run()
+	return a
+}
+
+func (a *Analytics) gatherData() *AnalyticsData {
+	return &AnalyticsData{
+		FunctionCallCount: atomic.LoadUint64(&globalFunctionCallCount),
+	}
+}
+
+func (a *Analytics) run() {
+	ticker := time.NewTicker(24 * time.Hour)
+	for range ticker.C {
+		msg := a.gatherData()
+		msg.id = a.id
+
+		msgbytes, err := json.Marshal(*msg)
+		if err != nil {
+			continue
+		}
+
+		_, _ = http.Post(a.url, "application/json", bytes.NewReader(msgbytes))
+	}
+}