Add throttle settings (throttle_attribute, node/shared throttle limit…

[LilyCaroline17]

Description

Adds throttle configuration fields to the WorkloadGroup construct as the foundational layer for WLM Request Throttling (# 21064). This PR introduces and persists the settings on the workload group construct.

Fields: Added 3 fields to the workload group construct

• throttle_attribute — the dimension the limit is keyed by (e.g. principal.username). Optional; when omitted, the whole group is throttled as a single bucket.
• node_throttle_limit — per-node in-flight allowance
• shared_throttle_limit — additional central pool drawn on after a node exhausts its local allowance.

All three are optional. The total cluster-wide ceiling is node_throttle_limit × N + shared_throttle_limit.

Example

PUT _wlm/workload_group/
{
  "name": "analytics",
  "resiliency_mode": "enforced",
  "resource_limits": { "cpu": 0.3, "memory": 0.4 },
  "throttle_attribute": "principal.username",
  "node_throttle_limit": 10,
  "shared_throttle_limit": 20
}

Update / clear semantics

Throttle fields follow three-way merge semantics on update:

• Field absent from the request → keep the existing value.
• Field present with a value → overwrite.
• Field present as explicit JSON null → clear the field (disable that dimension).

# To disable node-level throttling, keep the shared pool and attribute, can send:
PUT _wlm/workload_group/analytics
{ 
  "node_throttle_limit": null 
}
# Same logic applies to the shared_throttle_limit

Validation

• Limits must be non-negative when set (node_throttle_limit / shared_throttle_limit).
• throttle_attribute cannot be blank when set.
• A group with throttle_attribute must have at least one limit (nothing to enforce otherwise).
• If throttling is enabled, the effective ceiling (node ?? 0) + (shared ?? 0) must be ≥ 1 (a 0 ceiling would reject all requests).

throttle_attribute is currently stored as an opaque non-blank string; it is not yet validated against the WLM attribute registry (index_pattern, principal.username, principal.role). Will complete in a later PR

Testing

Added unit tests in WorkloadGroupTests. Manually verified end-to-end against a live single-node cluster (./gradlew :run -PinstalledPlugins="['workload-management']", WLM mode enabled): create, GET, merge-update, clear-via-null, and all four validation rejections behave as expected.

Related Issues

Resolves # 21064

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