allow overriding HttpResponse.getContent to install download throttler #586

protected Storage installHttpThrottler(StorageOptions.Builder storageOptions) {
        return storageOptions
                .setTransportOptions(HttpTransportOptions.newBuilder().setHttpTransportFactory(new ThrottledHttpTransportFactory()).build())
                .build()
                .getService();
    }

class ThrottledHttpTransportFactory implements HttpTransportFactory {
        public HttpTransport create() { return new NetHttpTransport.Builder().setConnectionFactory(new ThrottledConnectionFactory()).build(); }
    }

class ThrottledConnectionFactory extends DefaultConnectionFactory {
        @Override
        public HttpURLConnection openConnection(URL url) throws IOException {
            return new ThrottledUrlConnection(super.openConnection(url), url);
        }
    }

class ThrottledUrlConnection extends HttpURLConnection {
        HttpURLConnection delegate;

        ThrottledUrlConnection(HttpURLConnection delegate, URL url) {
            super(url);
            this.delegate = delegate;
        }

        @Override
        public InputStream getInputStream() throws IOException {
            return new ThrottledInputStream(delegate.getInputStream(), rateLimiter, throughputMeter);
        }
// followed by a long list of overridden methods to redirect via 'delegate'
....
}