Node.js: Is there a way to mock the prometheus pushgateway server?

So I have solved the problem by creating a simple http server.

The following is the code of the server:

const http = require('http');

const body = [];
let text = null;
let path = null;

function createServer(port) {
  return http.createServer()
    .on('request', (req, res) => {
      if (req.method === 'POST' || req.method === 'PUT') {
        path = req.url;
        req.on('data', (chunk) => {
          body.push(chunk);
        }).on('end', () => {
          text = Buffer.concat(body).toString();
          res.end(`${req.method} success`);
        }).on('error', (err) => {
          console.error(err);
        });
      } else if (req.method === 'GET') {
        res.end(JSON.stringify({ path, text }));
      } else if (req.method === 'DELETE') {
        path = null;
        text = null;
        res.end('DELETE success');
      }
    }).on('error', (err) => {
      console.log(`Server error: ${err}`);
    })
    .listen(port, '0.0.0.0');
}

module.exports = (createServer);

The server accepts POST/PUT/DELETE/GET requests, to handle the pushAdd()/push()/delete() functions of the gateway to handle the metric data, as well as checking the push data on the mocked pushgateway.

Also, the request.url and the text data pushed to the server are saved and passed to the test program for verification.

The following is one of the test cases (using Mocha+Chai):

describe('Check adding custom push count metric', () => {
  it('Check connection: should return - network status: 200, result contains custom count metric string', async () => {
    metricInstance = promMetric.createCustomPushMetric({
      name: 'test_counter',
      help: 'Used to test the pushgateway for custom counter metrics',
      type: 'counter',
      jobName: 'custom-metric-pushgateway-counter',
      groupings: { key: 'test', type: 'customMetric' },
    });
    await promMetric.addPushMetric(metricInstance, 0.879);
    await sleep(500);
    const result = await chai.request(mockPgwServer).get('/');
    expect(result).to.have.status(200);
    const json = JSON.parse(result.text);
    expect(json.path).to.equal('/metrics/job/custom-metric-pushgateway-counter/key/test/type/customMetric');
    expect(json.text).to.match(/# HELP Push_gateway_test_test_counter Used to test the pushgateway for custom counter metrics/);
    expect(json.text).to.match(/# TYPE Push_gateway_test_test_counter counter/);
    expect(json.text).to.match(/Push_gateway_test_test_counter 0.879/);
  });
});

The "path" and "text" stored in the server when the metric is posted from the client are sent back to the client when queried and verified.