nettrace: Add a new tracing library

This commit introduces a new tracing library, that replaces
golang.org/x/net/trace, and supports (amongts other thing) nested
traces.

This is a minimal change, future patches will make use of the new
functionality.

internal/nettrace/histogram.go

@@ -0,0 +1,71 @@
+package nettrace
+
+import (
+	"time"
+)
+
+type histogram struct {
+	count [nBuckets]uint64
+
+	totalQ uint64
+	totalT time.Duration
+	min    time.Duration
+	max    time.Duration
+}
+
+func (h *histogram) Add(bucket int, latency time.Duration) {
+	if h.totalQ == 0 || h.min > latency {
+		h.min = latency
+	}
+	if h.max < latency {
+		h.max = latency
+	}
+
+	h.count[bucket]++
+	h.totalQ++
+	h.totalT += latency
+}
+
+type histSnapshot struct {
+	Counts        map[time.Duration]line
+	Count         uint64
+	Avg, Min, Max time.Duration
+}
+
+type line struct {
+	Start     time.Duration
+	BucketIdx int
+	Count     uint64
+	Percent   float32
+	CumPct    float32
+}
+
+func (h *histogram) Snapshot() *histSnapshot {
+	s := &histSnapshot{
+		Counts: map[time.Duration]line{},
+		Count:  h.totalQ,
+		Min:    h.min,
+		Max:    h.max,
+	}
+
+	if h.totalQ > 0 {
+		s.Avg = time.Duration(uint64(h.totalT) / h.totalQ)
+	}
+
+	var cumCount uint64
+	for i := 0; i < nBuckets; i++ {
+		cumCount += h.count[i]
+		l := line{
+			Start:     buckets[i],
+			BucketIdx: i,
+			Count:     h.count[i],
+		}
+		if h.totalQ > 0 {
+			l.Percent = float32(h.count[i]) / float32(h.totalQ) * 100
+			l.CumPct = float32(cumCount) / float32(h.totalQ) * 100
+		}
+		s.Counts[buckets[i]] = l
+	}
+
+	return s
+}