feat: measure startup delay and journey duration

[vigneshshanmugam]

• Measures the startup time in epoch micros, which will be used by Heartbeat to measure the actual startup delay of the Node.js process.
• Also the browser launch delay process is measured and reported in micros.
• Also fixes Add journey.duration.us to the JSON spec #412 where journey.duration.us is reported.

Journey end event

{"type":"journey/end","@timestamp":1677539972109132.8,"journey":{"name":"journey-0","id":"journey-0","status":"succeeded","duration":{"us":199580}},"root_fields":{"browser_delay_us":57378,"process_startup_epoch_us":1677539969494005,"os":{"platform":"darwin"},"package":{"name":"@elastic/synthetics","version":"1.0.0-beta.40"}},"payload":{"status":"succeeded"},"package_version":"1.0.0-beta.40"}

[apmmachine]

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• Start Time: 2023-02-28T00:09:25.985+0000

• Duration: 14 min 9 sec

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