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/metrics.go

@@ -2,11 +2,14 @@ package router
 
 import (
 	"fmt"
+	"sync/atomic"
 	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
 )
 
+var globalFunctionCallCount uint64
+
 type (
 	// functionLabels is the set of metrics labels that relate to
 	// functions.
@@ -116,6 +119,8 @@ func labelsToStrings(f *functionLabels, h *httpLabels) []string {
 }
 
 func functionCallCompleted(f *functionLabels, h *httpLabels, overhead, duration time.Duration, respSize int64) {
+	atomic.AddUint64(&globalFunctionCallCount, 1)
+
 	l := labelsToStrings(f, h)
 
 	// overhead: time from request ingress into router upto proxing into function pod