diff --git a/vendor/github.com/rcrowley/go-metrics/ewma.go b/vendor/github.com/rcrowley/go-metrics/ewma.go
new file mode 100644
index 0000000..a8183dd
--- /dev/null
+++ b/vendor/github.com/rcrowley/go-metrics/ewma.go
@@ -0,0 +1,138 @@
+package metrics
+
+import (
+	"math"
+	"sync"
+	"sync/atomic"
+)
+
+// EWMAs continuously calculate an exponentially-weighted moving average
+// based on an outside source of clock ticks.
+type EWMA interface {
+	Rate() float64
+	Snapshot() EWMA
+	Tick()
+	Update(int64)
+}
+
+// NewEWMA constructs a new EWMA with the given alpha.
+func NewEWMA(alpha float64) EWMA {
+	if UseNilMetrics {
+		return NilEWMA{}
+	}
+	return &StandardEWMA{alpha: alpha}
+}
+
+// NewEWMA1 constructs a new EWMA for a one-minute moving average.
+func NewEWMA1() EWMA {
+	return NewEWMA(1 - math.Exp(-5.0/60.0/1))
+}
+
+// NewEWMA5 constructs a new EWMA for a five-minute moving average.
+func NewEWMA5() EWMA {
+	return NewEWMA(1 - math.Exp(-5.0/60.0/5))
+}
+
+// NewEWMA15 constructs a new EWMA for a fifteen-minute moving average.
+func NewEWMA15() EWMA {
+	return NewEWMA(1 - math.Exp(-5.0/60.0/15))
+}
+
+// EWMASnapshot is a read-only copy of another EWMA.
+type EWMASnapshot float64
+
+// Rate returns the rate of events per second at the time the snapshot was
+// taken.
+func (a EWMASnapshot) Rate() float64 { return float64(a) }
+
+// Snapshot returns the snapshot.
+func (a EWMASnapshot) Snapshot() EWMA { return a }
+
+// Tick panics.
+func (EWMASnapshot) Tick() {
+	panic("Tick called on an EWMASnapshot")
+}
+
+// Update panics.
+func (EWMASnapshot) Update(int64) {
+	panic("Update called on an EWMASnapshot")
+}
+
+// NilEWMA is a no-op EWMA.
+type NilEWMA struct{}
+
+// Rate is a no-op.
+func (NilEWMA) Rate() float64 { return 0.0 }
+
+// Snapshot is a no-op.
+func (NilEWMA) Snapshot() EWMA { return NilEWMA{} }
+
+// Tick is a no-op.
+func (NilEWMA) Tick() {}
+
+// Update is a no-op.
+func (NilEWMA) Update(n int64) {}
+
+// StandardEWMA is the standard implementation of an EWMA and tracks the number
+// of uncounted events and processes them on each tick.  It uses the
+// sync/atomic package to manage uncounted events.
+type StandardEWMA struct {
+	uncounted int64 // /!\ this should be the first member to ensure 64-bit alignment
+	alpha     float64
+	rate      uint64
+	init      uint32
+	mutex     sync.Mutex
+}
+
+// Rate returns the moving average rate of events per second.
+func (a *StandardEWMA) Rate() float64 {
+	currentRate := math.Float64frombits(atomic.LoadUint64(&a.rate)) * float64(1e9)
+	return currentRate
+}
+
+// Snapshot returns a read-only copy of the EWMA.
+func (a *StandardEWMA) Snapshot() EWMA {
+	return EWMASnapshot(a.Rate())
+}
+
+// Tick ticks the clock to update the moving average.  It assumes it is called
+// every five seconds.
+func (a *StandardEWMA) Tick() {
+	// Optimization to avoid mutex locking in the hot-path.
+	if atomic.LoadUint32(&a.init) == 1 {
+		a.updateRate(a.fetchInstantRate())
+	} else {
+		// Slow-path: this is only needed on the first Tick() and preserves transactional updating
+		// of init and rate in the else block. The first conditional is needed below because
+		// a different thread could have set a.init = 1 between the time of the first atomic load and when
+		// the lock was acquired.
+		a.mutex.Lock()
+		if atomic.LoadUint32(&a.init) == 1 {
+			// The fetchInstantRate() uses atomic loading, which is unecessary in this critical section
+			// but again, this section is only invoked on the first successful Tick() operation.
+			a.updateRate(a.fetchInstantRate())
+		} else {
+			atomic.StoreUint32(&a.init, 1)
+			atomic.StoreUint64(&a.rate, math.Float64bits(a.fetchInstantRate()))
+		}
+		a.mutex.Unlock()
+	}
+}
+
+func (a *StandardEWMA) fetchInstantRate() float64 {
+	count := atomic.LoadInt64(&a.uncounted)
+	atomic.AddInt64(&a.uncounted, -count)
+	instantRate := float64(count) / float64(5e9)
+	return instantRate
+}
+
+func (a *StandardEWMA) updateRate(instantRate float64) {
+	currentRate := math.Float64frombits(atomic.LoadUint64(&a.rate))
+	currentRate += a.alpha * (instantRate - currentRate)
+	atomic.StoreUint64(&a.rate, math.Float64bits(currentRate))
+}
+
+// Update adds n uncounted events.
+func (a *StandardEWMA) Update(n int64) {
+	atomic.AddInt64(&a.uncounted, n)
+}