Add backoff package and fix Consul CPU usage (#635)

* Add backoff package

Justification for jitter and growth factor:
https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/.

Add backoff to the Consul instancer loop.

Fixes go-kit/kit#627.

* Revert "Add backoff package"

This reverts commit 924501ae1fcfadaa27593e9c019283412c513928.

* Get rid of external package and update exponential

* Add instancer backoff

* Fix old exponential name

* Add doc comment

* Fixup & respond to review

sd/consul/instancer.go

@@ -3,12 +3,14 @@ package consul
 import (
 	"fmt"
 	"io"
+	"time"
 
 	consul "github.com/hashicorp/consul/api"
 
 	"github.com/go-kit/kit/log"
 	"github.com/go-kit/kit/sd"
 	"github.com/go-kit/kit/sd/internal/instance"
+	"github.com/go-kit/kit/util/conn"
 )
 
 const defaultIndex = 0
@@ -59,6 +61,7 @@ func (s *Instancer) loop(lastIndex uint64) {
 	var (
 		instances []string
 		err       error
+		d         time.Duration = 10 * time.Millisecond
 	)
 	for {
 		instances, lastIndex, err = s.getInstances(lastIndex, s.quitc)
@@ -67,9 +70,12 @@ func (s *Instancer) loop(lastIndex uint64) {
 			return // stopped via quitc
 		case err != nil:
 			s.logger.Log("err", err)
+			time.Sleep(d)
+			d = conn.Exponential(d)
 			s.cache.Update(sd.Event{Err: err})
 		default:
 			s.cache.Update(sd.Event{Instances: instances})
+			d = 10 * time.Millisecond
 		}
 	}
 }

util/conn/manager.go

@@ -2,6 +2,7 @@ package conn
 
 import (
 	"errors"
+	"math/rand"
 	"net"
 	"time"
 
@@ -103,7 +104,7 @@ func (m *Manager) loop() {
 		case conn = <-connc:
 			if conn == nil {
 				// didn't work
-				backoff = exponential(backoff) // wait longer
+				backoff = Exponential(backoff) // wait longer
 				reconnectc = m.after(backoff)  // try again
 			} else {
 				// worked!
@@ -132,12 +133,18 @@ func dial(d Dialer, network, address string, logger log.Logger) net.Conn {
 	return conn
 }
 
-func exponential(d time.Duration) time.Duration {
+// Exponential takes a duration and returns another one that is twice as long, +/- 50%. It is
+// used to provide backoff for operations that may fail and should avoid thundering herds.
+// See https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/ for rationale
+func Exponential(d time.Duration) time.Duration {
 	d *= 2
+	jitter := rand.Float64() + 0.5
+	d = time.Duration(int64(float64(d.Nanoseconds()) * jitter))
 	if d > time.Minute {
 		d = time.Minute
 	}
 	return d
+
 }
 
 // ErrConnectionUnavailable is returned by the Manager's Write method when the