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/context_test.go

@@ -0,0 +1,66 @@
+package nettrace
+
+import (
+	"context"
+	"testing"
+)
+
+func TestContext(t *testing.T) {
+	tr := New("TestContext", "trace")
+	defer tr.Finish()
+
+	// Attach the trace to a new context.
+	ctx := NewContext(context.Background(), tr)
+
+	// Get the trace back from the context.
+	{
+		tr2, ok := FromContext(ctx)
+		if !ok {
+			t.Errorf("Context with trace returned not found")
+		}
+		if tr != tr2 {
+			t.Errorf("Trace from context is different: %v != %v", tr, tr2)
+		}
+	}
+
+	// Create a child trace from the context.
+	{
+		tr3 := ChildFromContext(ctx, "TestContext", "child")
+		if p := tr3.(*trace).Parent; p != tr {
+			t.Errorf("Child doesn't have the right parent: %v != %v", p, tr)
+		}
+		tr3.Finish()
+	}
+
+	// FromContextOrNew returns the one from the context.
+	{
+		tr4, ctx4 := FromContextOrNew(ctx, "TestContext", "from-ctx")
+		if ctx4 != ctx {
+			t.Errorf("Got new context: %v != %v", ctx4, ctx)
+		}
+		if tr4 != tr {
+			t.Errorf("Context with trace returned new trace: %v != %v", tr4, tr)
+		}
+	}
+
+	// FromContextOrNew needs to create a new one.
+	{
+		tr5, ctx5 := FromContextOrNew(
+			context.Background(), "TestContext", "tr5")
+		if tr, _ := FromContext(ctx5); tr != tr5 {
+			t.Errorf("Context with trace returned the wrong trace: %v != %v",
+				tr, tr5)
+		}
+		tr5.Finish()
+	}
+
+	// Child from a context that has no trace attached.
+	{
+		tr6 := ChildFromContext(
+			context.Background(), "TestContext", "child")
+		tr6.Finish()
+		if p := tr6.(*trace).Parent; p != nil {
+			t.Errorf("Expected orphan trace, it has a parent: %v", p)
+		}
+	}
+}