[o11y] Introduce distinct event type for STW events

[fhanau]

This will make it easier to distinguish between STW and BTW invocations internally, each event type should have a distinct type. The authoritative internal definition has already been updated for this.

Also see the internal PR, especially to verify that the right event type is being used.

[ask-bonk]

UnknownError: ProviderInitError

github run

[ask-bonk]

@fhanau Bonk workflow failed. Check the logs for details.

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[codspeed-hq]

Merging this PR will not alter performance

✅ 72 untouched benchmarks
⏩ 129 skipped benchmarks¹

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_{Comparing felix/041626-stw-event-type (19a302d) with main (45c1208)}

Footnotes

1. 129 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[jasnell]

Looks like there are legitmate (non-flaky) CI failures with this change in the internal project.

[fhanau]

Looks like there are legitmate (non-flaky) CI failures with this change in the internal project.

Yup, that's because I opened the workerd PR before the internal PR – the internal PR makes some test changes, CI is passing there