Is it possible to provide setTraceId() to actually customise and replace the traceId generated by the agent?

**Is your feature request related to a problem? Please describe.**

The current trace.id can only be generated by the agent and cannot establish the trace chain starting from the client.

**Describe the solution you'd like**

const apm = require('elastic-apm-node');
apm.setTraceId('customTraceId')