Improve Timing of Promise "then"

[k2la]

Currently, To measure latency of sending messages, asynchronous processing using Nodejs Promise like following code:

    const results = [];
    for (let mi = 0; mi < numOfMessages; mi += 1) {
      const startSendTimeMilliSeconds = performance.now();
      results.push(producer.send({
        data: message,
      }).then(() => {
        // add latency
        histogram.recordValue((performance.now() - startSendTimeMilliSeconds));
      }));
    }
    await Promise.all(results); // wait until all messages are sent.

https://github.com/apache/pulsar-client-node/blob/master/perf/perf_producer.js#L88-L98

This code creates histogram about latency of sending messages. However, all then() of Promise in results start after await Promise.all(results). So, We cannot get accurate histogram.