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.go

@@ -0,0 +1,40 @@
+package nettrace
+
+import "context"
+
+type ctxKeyT string
+
+const ctxKey ctxKeyT = "blitiri.com.ar/go/srv/nettrace"
+
+// NewContext returns a new context with the given trace attached.
+func NewContext(ctx context.Context, tr Trace) context.Context {
+	return context.WithValue(ctx, ctxKey, tr)
+}
+
+// FromContext returns the trace attached to the given context (if any).
+func FromContext(ctx context.Context) (Trace, bool) {
+	tr, ok := ctx.Value(ctxKey).(Trace)
+	return tr, ok
+}
+
+// FromContextOrNew returns the trace attached to the given context, or a new
+// trace if there is none.
+func FromContextOrNew(ctx context.Context, family, title string) (Trace, context.Context) {
+	tr, ok := FromContext(ctx)
+	if ok {
+		return tr, ctx
+	}
+
+	tr = New(family, title)
+	return tr, NewContext(ctx, tr)
+}
+
+// ChildFromContext returns a new trace that is a child of the one attached to
+// the context (if any).
+func ChildFromContext(ctx context.Context, family, title string) Trace {
+	parent, ok := FromContext(ctx)
+	if ok {
+		return parent.NewChild(family, title)
+	}
+	return New(family, title)
+}