Probably something like:

    // h already has a context.Context in scope through similar
    func MyHandler (ctx context.Context, h http.Handler) http.Handler {
       return http.HandlerFunc(func (rw http.ResponseWriter, req *http.Request) {
           // do stuff before handler
           h.serveHttp(rw, req)
           // do stuff after handler
       })
    }

then you can just chain your handlers and pass through a shared context