[Sampling] add get call for sampling configs

[seanzatzdev]

Add GET endpoint for sampling config:
e.g.
Get Configuration
The GET /index-name/_sample/config API will get a specific configuration. For example:

GET /logs/_sample/config,
returns

  {
    "index": "logs",
    "configuration:
    {
      "rate": ".05",
      "if": "ctx?.network?.name == 'Guest'"
    }
  }

[Copilot]

Pull Request Overview

This PR adds a GET endpoint to retrieve sampling configuration for a specific index. The implementation follows the existing patterns established by the PUT and DELETE operations for sampling configurations.

Key Changes

• Introduces GetSampleConfigurationAction with request/response classes for retrieving sampling config
• Implements REST and transport layer handlers for the new GET endpoint at /{index}/_sample/config
• Adds comprehensive test coverage including unit tests, integration tests, and REST API spec tests

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Constants.java	Adds the new action permission string for the GET operation
GetSampleConfigurationActionResponseTests.java	Unit tests for response serialization and mutation
GetSampleConfigurationActionRequestTests.java	Unit tests for request validation and behavior
TransportGetSampleConfigurationAction.java	Core transport action that retrieves config from cluster metadata
RestGetSampleConfigurationAction.java	REST handler for the GET endpoint
GetSampleConfigurationAction.java	Action definition with Request/Response classes
ActionModule.java	Registers the new action and REST handler
GetSampleConfigurationActionIT.java	Integration tests for various GET scenarios
10_basic.yml	REST API specification tests
indices.get_sample_configuration.json	REST API endpoint specification

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[masseyke]

I think we want this to be "indices:monitor/sample/config/get"

[seanzatzdev]

gotcha, does that influence anything behind the scenes or is it just a naming convention thing?

[masseyke]

The permissions are based off of it. We want fetching the config to only require the index monitor permission (vs index admin to create or modify or delete it).

[masseyke]

Same as above, I think this needs to be monitor.

[masseyke]

LGTM