diff --git a/vendor/github.com/rcrowley/go-metrics/histogram.go b/vendor/github.com/rcrowley/go-metrics/histogram.go
new file mode 100644
index 0000000..dbc837f
--- /dev/null
+++ b/vendor/github.com/rcrowley/go-metrics/histogram.go
@@ -0,0 +1,202 @@
+package metrics
+
+// Histograms calculate distribution statistics from a series of int64 values.
+type Histogram interface {
+	Clear()
+	Count() int64
+	Max() int64
+	Mean() float64
+	Min() int64
+	Percentile(float64) float64
+	Percentiles([]float64) []float64
+	Sample() Sample
+	Snapshot() Histogram
+	StdDev() float64
+	Sum() int64
+	Update(int64)
+	Variance() float64
+}
+
+// GetOrRegisterHistogram returns an existing Histogram or constructs and
+// registers a new StandardHistogram.
+func GetOrRegisterHistogram(name string, r Registry, s Sample) Histogram {
+	if nil == r {
+		r = DefaultRegistry
+	}
+	return r.GetOrRegister(name, func() Histogram { return NewHistogram(s) }).(Histogram)
+}
+
+// NewHistogram constructs a new StandardHistogram from a Sample.
+func NewHistogram(s Sample) Histogram {
+	if UseNilMetrics {
+		return NilHistogram{}
+	}
+	return &StandardHistogram{sample: s}
+}
+
+// NewRegisteredHistogram constructs and registers a new StandardHistogram from
+// a Sample.
+func NewRegisteredHistogram(name string, r Registry, s Sample) Histogram {
+	c := NewHistogram(s)
+	if nil == r {
+		r = DefaultRegistry
+	}
+	r.Register(name, c)
+	return c
+}
+
+// HistogramSnapshot is a read-only copy of another Histogram.
+type HistogramSnapshot struct {
+	sample *SampleSnapshot
+}
+
+// Clear panics.
+func (*HistogramSnapshot) Clear() {
+	panic("Clear called on a HistogramSnapshot")
+}
+
+// Count returns the number of samples recorded at the time the snapshot was
+// taken.
+func (h *HistogramSnapshot) Count() int64 { return h.sample.Count() }
+
+// Max returns the maximum value in the sample at the time the snapshot was
+// taken.
+func (h *HistogramSnapshot) Max() int64 { return h.sample.Max() }
+
+// Mean returns the mean of the values in the sample at the time the snapshot
+// was taken.
+func (h *HistogramSnapshot) Mean() float64 { return h.sample.Mean() }
+
+// Min returns the minimum value in the sample at the time the snapshot was
+// taken.
+func (h *HistogramSnapshot) Min() int64 { return h.sample.Min() }
+
+// Percentile returns an arbitrary percentile of values in the sample at the
+// time the snapshot was taken.
+func (h *HistogramSnapshot) Percentile(p float64) float64 {
+	return h.sample.Percentile(p)
+}
+
+// Percentiles returns a slice of arbitrary percentiles of values in the sample
+// at the time the snapshot was taken.
+func (h *HistogramSnapshot) Percentiles(ps []float64) []float64 {
+	return h.sample.Percentiles(ps)
+}
+
+// Sample returns the Sample underlying the histogram.
+func (h *HistogramSnapshot) Sample() Sample { return h.sample }
+
+// Snapshot returns the snapshot.
+func (h *HistogramSnapshot) Snapshot() Histogram { return h }
+
+// StdDev returns the standard deviation of the values in the sample at the
+// time the snapshot was taken.
+func (h *HistogramSnapshot) StdDev() float64 { return h.sample.StdDev() }
+
+// Sum returns the sum in the sample at the time the snapshot was taken.
+func (h *HistogramSnapshot) Sum() int64 { return h.sample.Sum() }
+
+// Update panics.
+func (*HistogramSnapshot) Update(int64) {
+	panic("Update called on a HistogramSnapshot")
+}
+
+// Variance returns the variance of inputs at the time the snapshot was taken.
+func (h *HistogramSnapshot) Variance() float64 { return h.sample.Variance() }
+
+// NilHistogram is a no-op Histogram.
+type NilHistogram struct{}
+
+// Clear is a no-op.
+func (NilHistogram) Clear() {}
+
+// Count is a no-op.
+func (NilHistogram) Count() int64 { return 0 }
+
+// Max is a no-op.
+func (NilHistogram) Max() int64 { return 0 }
+
+// Mean is a no-op.
+func (NilHistogram) Mean() float64 { return 0.0 }
+
+// Min is a no-op.
+func (NilHistogram) Min() int64 { return 0 }
+
+// Percentile is a no-op.
+func (NilHistogram) Percentile(p float64) float64 { return 0.0 }
+
+// Percentiles is a no-op.
+func (NilHistogram) Percentiles(ps []float64) []float64 {
+	return make([]float64, len(ps))
+}
+
+// Sample is a no-op.
+func (NilHistogram) Sample() Sample { return NilSample{} }
+
+// Snapshot is a no-op.
+func (NilHistogram) Snapshot() Histogram { return NilHistogram{} }
+
+// StdDev is a no-op.
+func (NilHistogram) StdDev() float64 { return 0.0 }
+
+// Sum is a no-op.
+func (NilHistogram) Sum() int64 { return 0 }
+
+// Update is a no-op.
+func (NilHistogram) Update(v int64) {}
+
+// Variance is a no-op.
+func (NilHistogram) Variance() float64 { return 0.0 }
+
+// StandardHistogram is the standard implementation of a Histogram and uses a
+// Sample to bound its memory use.
+type StandardHistogram struct {
+	sample Sample
+}
+
+// Clear clears the histogram and its sample.
+func (h *StandardHistogram) Clear() { h.sample.Clear() }
+
+// Count returns the number of samples recorded since the histogram was last
+// cleared.
+func (h *StandardHistogram) Count() int64 { return h.sample.Count() }
+
+// Max returns the maximum value in the sample.
+func (h *StandardHistogram) Max() int64 { return h.sample.Max() }
+
+// Mean returns the mean of the values in the sample.
+func (h *StandardHistogram) Mean() float64 { return h.sample.Mean() }
+
+// Min returns the minimum value in the sample.
+func (h *StandardHistogram) Min() int64 { return h.sample.Min() }
+
+// Percentile returns an arbitrary percentile of the values in the sample.
+func (h *StandardHistogram) Percentile(p float64) float64 {
+	return h.sample.Percentile(p)
+}
+
+// Percentiles returns a slice of arbitrary percentiles of the values in the
+// sample.
+func (h *StandardHistogram) Percentiles(ps []float64) []float64 {
+	return h.sample.Percentiles(ps)
+}
+
+// Sample returns the Sample underlying the histogram.
+func (h *StandardHistogram) Sample() Sample { return h.sample }
+
+// Snapshot returns a read-only copy of the histogram.
+func (h *StandardHistogram) Snapshot() Histogram {
+	return &HistogramSnapshot{sample: h.sample.Snapshot().(*SampleSnapshot)}
+}
+
+// StdDev returns the standard deviation of the values in the sample.
+func (h *StandardHistogram) StdDev() float64 { return h.sample.StdDev() }
+
+// Sum returns the sum in the sample.
+func (h *StandardHistogram) Sum() int64 { return h.sample.Sum() }
+
+// Update samples a new value.
+func (h *StandardHistogram) Update(v int64) { h.sample.Update(v) }
+
+// Variance returns the variance of the values in the sample.
+func (h *StandardHistogram) Variance() float64 { return h.sample.Variance() }