Improve Timing of Promise "then"

Currently, To measure latency of sending messages, asynchronous processing using Nodejs [Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/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.

Start with perf/perf_producer.js around lines 88-98 and review how producer.send() promises and Promise.all() are used. Verify when each then() callback runs relative to Promise.all(), then make the latency histogram reflect the actual message-send timing and confirm the performance script still completes correctly.

Written by the indexing model from the issue text.