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_test.go

@@ -0,0 +1,29 @@
+package nettrace
+
+import (
+	"testing"
+	"time"
+)
+
+func TestHistogramBasic(t *testing.T) {
+	h := histogram{}
+	h.Add(1, 1*time.Millisecond)
+
+	snap := h.Snapshot()
+	if snap.Count != 1 ||
+		snap.Min != 1*time.Millisecond ||
+		snap.Max != 1*time.Millisecond ||
+		snap.Avg != 1*time.Millisecond {
+		t.Errorf("expected snapshot with only 1 sample, got %v", snap)
+	}
+}
+
+func TestHistogramEmpty(t *testing.T) {
+	h := histogram{}
+	snap := h.Snapshot()
+
+	if len(snap.Counts) != nBuckets || snap.Count != 0 ||
+		snap.Avg != 0 || snap.Min != 0 || snap.Max != 0 {
+		t.Errorf("expected zero snapshot, got %v", snap)
+	}
+}