use Date.now and tick offset to calculate span time

[ngraef]

📝 Description

Fixes span timing inconsistencies across modules by using Date.now() for startTime and perf_hooks to measure duration.

🎯 Relevant issues

Fixes #897

💎 Type of change

• Bug fix (non-breaking change which fixes an issue)

🚦 How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Unit tests
• Integration tests
• Tested with 5 distributed services sending to Datadog

🏁 Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have commented my code, particularly in hard-to-understand areas

[icebob]

Good job. Please remove the unused now.js" module.