PPI-165 : fix timestamp rounding issues and expose helpers for high res timestamps

[blakeroberts-wk]

Which problem is this PR solving?

WebTimeProvider suffers from rounding issues when using window.performance.timeOrigin:

raw (ms) 1722292170313.4
cur (ns) 1722292170313399800

The output above comes from the following script:

// import 'dart:html';

const int _nanosecondsPerMillisecond = 1000 * 1000;

void main() {
  final raw = 1722292170313.4;
  print('raw (ms) $raw');

  final cur = (raw * _nanosecondsPerMillisecond).round();
  print('cur (ns) ${cur}');

//   final raw = window.performance.timeOrigin ??
//       window.performance.timing.navigationStart;
//   final cur = (raw * _nanosecondsPerMillisecond).round();
//   print('raw now (ms) ${raw}');
//   print('cur now (ns) ${cur}');
}

Short description of the change

Update conversion from doubles representing milliseconds to int64s representing nanoseconds.

Additionally, experimental helper methods have been exposed from the web_sdk package to assist with using timestamps from the performance API as timestamps in OpenTelemetry (span start time, end time, span event timestamps).

How Has This Been Tested?

Used dart pad, unit tests, and an example app which created spans from performance entry timestamps.

Checklist:

• Unit tests have been added
• Documentation has been updated

[aviary3-wk]

Security Insights

No security relevant content was detected by automated scans.

Action Items

• Review PR for security impact; comment "security review required" if needed or unsure
• Verify aviary.yaml coverage of security relevant code

---

Questions or Comments? Reach out on Slack: #support-infosec.

[blakeroberts-wk]

QA +1 unit tests and dart pad produce expected output. additional manual verification was used within a browser app where spans were created using timeOrigin as well as timestamps from performance events/marks

[blakeroberts-wk]

@Workiva/release-management-p

[rmconsole-wf]

+1 from RM