feat(slo): Consider empty slice as good slice for sli calculation purposes on timeslice budgeting method

[kdelemme]

Resolves https://github.com/elastic/kibana/issues/181577

🏎️ Summary

We used to calculate the SLI value of Timeslices SLOs based on the observed good and total events. The good events was the sum of isGoodSlice, while the total events was the value_count of isGoodSlice. When there is no data, the rollup documents are not created, and therefore the good and total values are not taking these slices into account.

This PR changes this behaviour. For a Timeslices SLO, we always use the total slices available in the time window, e.g. 10080 for a 7d rolling or weekly calendar aligned. And we compute the SLI as 1 - (total - good) / total_slices. We do this change in the different services that fetch the SLI rollup documents in order to calculate the SLI value. And also in the summary transform for Timeslices SLO.

Scenario	Good Events (observed)	Total Events (observed)	Bad Events (observed)	Total Slices In Period	SLI (old)	SLI (new)
7d rolling, 1m slice, no missing data	10000	10080	80	10080	0.99206349	0.99206349
7d rolling, 60m slice, no missing data	158	168	10	168	0.94047619	0.94047619
7d rolling, 1m slice, missing data	100	120	20	10080	0.83333333	0.99801587

Release note

Timeslice SLOs calculation for the sli value changed to include the no data slices as good slice.

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Public APIs missing comments

Total count of every public API that lacks a comment. Target amount is 0. Run node scripts/build_api_docs --plugin [yourplugin] --stats comments for more detailed information.

id	before	after	diff
@kbn/slo-schema	179	178	-1

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
slo	726.3KB	726.3KB	-57.0B

Canvas Sharable Runtime

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id	before	after	diff
module count	-	5871	+5871
total size	-	6.7MB	+6.7MB

Page load bundle

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id	before	after	diff
observability	151.1KB	151.1KB	-60.0B

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id	before	after	diff
@kbn/slo-schema	179	178	-1

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[kdelemme]

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Status	Branch	Result
✅	8.14

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