How can I create traceId and start/end time when using elastic APM API?

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I am using https://github.com/elastic/apm-agent-nodejs in a node application to send trace data to Elastic APM Server.
```
const getApm = (serviceName = 'my-service') =>
require('elastic-apm-node').start({
serviceName,
serverUrl:
'server url',
});

const apm = getApm();
apm.startTransaction('transaction1');
apm.startSpan(...)
```
I will send trace data to APM server and I am able to view them via Kibana. The trace id, start/end time is calculated by APM client or server. H

**Describe the solution you'd like**
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I'd like to use the API in this way:

```
apm.startTransaction('name', { startTime: xxx, endTime: xxx, traceId})

apm.startSpan('name', { startTime: xxx, endTime: xxx})
...
```

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