tx.go
func TraceExternal(ctx context.Context, host, name string) *Trace {
	return trace(ctx, name, func(tx Tx) error {
		return tx.StartExternal(host, name)
	})
}
func TraceExternal(ctx context.Context, host, name string) *Trace {
	return trace(ctx, name, func(tx Tx) error {
		return tx.StartExternal(host, name)
	})
}
// TraceExternal adds an external segment to the newrelic transaction, if one exists in the context.