swarm: integrate OpenTracing; propagate ctx to internal APIs (#17169)

* swarm: propagate ctx, enable opentracing

* swarm/tracing: log error when tracing is misconfigured

vendor/github.com/codahale/hdrhistogram/window.go

@@ -0,0 +1,45 @@
+package hdrhistogram
+
+// A WindowedHistogram combines histograms to provide windowed statistics.
+type WindowedHistogram struct {
+	idx int
+	h   []Histogram
+	m   *Histogram
+
+	Current *Histogram
+}
+
+// NewWindowed creates a new WindowedHistogram with N underlying histograms with
+// the given parameters.
+func NewWindowed(n int, minValue, maxValue int64, sigfigs int) *WindowedHistogram {
+	w := WindowedHistogram{
+		idx: -1,
+		h:   make([]Histogram, n),
+		m:   New(minValue, maxValue, sigfigs),
+	}
+
+	for i := range w.h {
+		w.h[i] = *New(minValue, maxValue, sigfigs)
+	}
+	w.Rotate()
+
+	return &w
+}
+
+// Merge returns a histogram which includes the recorded values from all the
+// sections of the window.
+func (w *WindowedHistogram) Merge() *Histogram {
+	w.m.Reset()
+	for _, h := range w.h {
+		w.m.Merge(&h)
+	}
+	return w.m
+}
+
+// Rotate resets the oldest histogram and rotates it to be used as the current
+// histogram.
+func (w *WindowedHistogram) Rotate() {
+	w.idx++
+	w.Current = &w.h[w.idx%len(w.h)]
+	w.Current.Reset()
+}