fix: Correct event timestamp values in server language templates (DENG-10432)

[sean-rose]

DENG-10432: Event timestamps in events stream for Glean server apps are consistently wrong

Per the Glean docs:

Each event contains the following data:

• A timestamp, in milliseconds. The first event in any ping always has a value of 0, and subsequent event timestamps are relative to it.

Pull Request checklist

• Quality: This PR builds and tests run cleanly
  • make test runs without emitting any warnings
  • make lint runs without emitting any errors
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry to CHANGELOG.md or an explanation of why it does not need one
  • Any breaking changes to language binding APIs are noted explicitly