[OpenAPI][SLO] Add discriminator to timeslice metrics types

[lloydmeta]

Summary

This PR adds a discriminator to the SLO timeslice metric types in the OpenAPI specification. The discriminator is introduced to address a critical issue encountered with client generation and data marshalling; specifically, when handling polymorphic metric types like percentile, which is a superset of the basic field metric type. Without a discriminator, clients (such as the Terraform provider) will incorrectly unmarshal data, defaulting fields like percentile to zero due to ambiguous type resolution.

Why is this needed?

• OpenAPI code generation: When generating clients from the OpenAPI schema, the lack of a discriminator can cause unmarshalling logic to use first-type-match wins. For example, metrics intended to be of the percentile type are instead parsed as the basic field metric type, losing the percentile field (see discussion). This leads to incorrect default values and change detection issues in downstream systems.
• Correct type mapping: The discriminator allows reliable mapping
• The Elastic Stack TF provider change has been merged with this fix anyway - not having this in the upstream harms Kibana clients and in addition, will break the provider if they pull in new versions of your spec without this fix

Checklist

Check the PR satisfies following conditions.

Reviewers should verify this PR satisfies this list as well.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines
• Review the backport guidelines and apply applicable backport:* labels.

Identify risks

n/na

Related

• Add support for timeslice metrics indicator terraform-provider-elasticstack#1195 (comment)
• [Feature] Timeslice metric indicator for elasticstack_kibana_slo (supporting percentile aggregations) terraform-provider-elasticstack#814

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)

[cla-checker-service]

💚 CLA has been signed

[simianhacker]

Thanks for this contribution! To help with the review process and make this change easier for other contributors to understand, could you add a bit more context to the PR description? It would be helpful to know "What problem this this PR solves or the issues you are running into?"

This additional context will make it much easier for reviewers to understand the motivation and impact of the change. Thanks!

[lloydmeta]

This additional context will make it much easier for reviewers to understand the motivation and impact of the change. Thanks!

Done.

Also note that there is some urgency here in the last point I added to the description

• The Elastic Stack TF provider change has been merged with this fix anyway - not having this in the upstream harms Kibana clients and in addition, will break the provider if they pull in new versions of your spec without this fix

[florent-leborgne]

Docs change LGTM

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 3b7ee39

Metrics [docs]

✅ unchanged

History

• 💛 Build #319052 was flaky a28da5c
• 💚 Build #317655 succeeded a9ae551

[lloydmeta]

@elastic/obs-ux-management-team @elastic/core-docs your review is needed to get this merged (kind reminder there is some urgency) 🙏🏼

[shahzad31]

LGTM !!