[Cases] Add MTTR metric

[cnasikas]

Summary

This PR adds the MTTR (mean time to response) metric for all cases. It follows the same pattern as with the single case metrics.

Depends on #130448

To verify

1. Create a case and leave it open for a while.
2. Close the case.
3. Do the following request: GET /api/cases/metrics/?features=["mttr"]

The endpoint accepts the following query parameters:

• owner: string | string[]
• from: an ISO date
• to: an ISO date

Try different values to verify that the endpoint respects the query parameters.

Checklist

Delete any items that are not applicable to this PR.

• Unit or functional tests were updated or added to match the most common scenarios

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[elasticmachine]

Pinging @elastic/response-ops-cases (Feature:Cases)

[cnasikas]

@elasticmachine merge upstream

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 1b742f9
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• Webpack Bundle Analyzer

Metrics [docs]

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
cases	91.1KB	91.3KB	+234.0B

History

• 💛 Build #41520 was flaky 156db5d
• 💔 Build #41509 failed 8ef0958
• 💔 Build #40884 failed f55307b
• 💔 Build #38705 failed 679fa7c
• 💔 Build #38687 failed 67cd8ad

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @cnasikas