feat: Created specs for ltr plugin for stats endpoints

[JohannesDaniel]

Description

Adds API specs for the stats endpoints of the LTR plugin.

Issues Resolved

The PR solves only parts of the subsequent issue to keep its size reasonable.
#786

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

Changes Analysis

Commit SHA: 3d9be01
Comparing To SHA: 28f4510

API Changes

Summary

├─┬Paths
│ ├──[➕] path (4655:3)
│ ├──[➕] path (4638:3)
│ ├──[➕] path (4689:3)
│ └──[➕] path (4673:3)
└─┬Components
  ├──[➕] responses (32079:7)
  ├──[➕] parameters (23813:7)
  ├──[➕] parameters (23827:7)
  ├──[➕] parameters (23803:7)
  ├──[➕] schemas (56569:7)
  ├──[➕] schemas (56525:7)
  ├──[➕] schemas (56558:7)
  ├──[➕] schemas (56512:7)
  ├──[➕] schemas (56534:7)
  └──[➕] schemas (56543:7)

Document Element	Total Changes	Breaking Changes
components	10	0
paths	4	0

• Total Changes: 14
• Additions: 14

Report

The full API changes report is available at: https://github.com/opensearch-project/opensearch-api-specification/actions/runs/13815195823/artifacts/2738703945

API Coverage

	Before	After	Δ
Covered (%)	663 (64.94 %)	663 (64.94 %)	0 (0 %)
Uncovered (%)	358 (35.06 %)	358 (35.06 %)	0 (0 %)
Unknown	45	49	4

[JohannesDaniel]

can someone help me solving this merge-coverage issue? I have not understood what needs to be done there

[dblock]

can someone help me solving this merge-coverage issue? I have not understood what needs to be done there

It's saying that tests/plugins/learning_to_rank/ltr/stats.yaml isn't actually used. You actually have to add it to the matrix.

(please do improve the error message!)

[github-actions]

Spec Test Coverage Analysis

Total	Tested
599	597 (99.67 %)