Promote `CountsAndLists` Feature Gate from Beta to Stable

What does this feature do?

CountsAndLists provides generic integer counter and string-list tracking on GameServers. It enables:

• Declaring named counters (integer) and lists (up to 1000 string values) on a GameServer at creation time
• Reading/updating those values via the SDK (GetCounter, UpdateCounter, GetList, UpdateList, AddListValue, RemoveListValue)
• Filtering GameServerAllocations based on counter/list capacity and priorities
• Fleet auto-scaling driven by available counter/list capacity
• Aggregated counter/list status rolled up to Fleet and GameServerSet

All SDK methods for this feature live in the beta SDK packages today (Go: sdks/go/beta.go, C#: sdks/csharp/sdk/Beta.cs, Rust: sdks/rust/src/beta.rs, Unity: sdks/unity/AgonesBetaSdk.cs).

Current state: Beta (enabled by default since v1.41.0)

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⚠️ This is a large, cross-cutting change. It should be broken into multiple PRs to keep reviews manageable. A suggested breakdown is at the bottom of this issue.

Acceptance Criteria / Tasks

• Manual audit: Before starting, grep the repo for FeatureCountsAndLists and CountsAndLists to confirm all usage sites (~378 occurrences today)

Feature gate removal

• API validation — Remove FeatureCountsAndLists guards; fold to always-permitted:
  • pkg/apis/agones/v1/gameserver.go — allow Counters/Lists fields unconditionally
  • pkg/apis/agones/v1/fleet.go — allow Priorities unconditionally
  • pkg/apis/agones/v1/gameserverset.go — allow Priorities unconditionally
  • pkg/apis/autoscaling/v1/fleetautoscaler.go — allow CounterPolicy/ListPolicy unconditionally
  • pkg/apis/allocation/v1/gameserverallocation.go — allow Counters/Lists/Priorities unconditionally
• Controllers — Always aggregate counter/list status:
  • pkg/gameserversets/controller.go
  • pkg/fleets/controller.go
• Allocation — Remove gates from counter/list matching, sorting, and cache population:
  • pkg/gameserverallocations/allocator.go, allocation_cache.go, find.go, converter.go
• Fleet autoscaler — pkg/fleetautoscalers/fleetautoscalers.go — remove CounterPolicy/ListPolicy gate
• Sorting — pkg/gameserversets/gameserversets.go — remove priority-sorting gate
• Metrics — pkg/metrics/controller.go — remove counter metric gate
• SDK Server — Remove feature gate checks from all Counter/List RPC handlers in pkg/sdkserver/sdkserver.go and pkg/sdkserver/localsdk.go

SDK promotion

Per the Agones SDK stability policy: when a Beta SDK feature graduates to Stable at release X, the Beta surface is marked deprecated at release X and removed 5 releases later.

• Copy Counter/List RPC definitions from proto/sdk/beta/beta.proto into proto/sdk/sdk.proto — do not remove them from beta.proto yet
• Run make gen-all-sdk-grpc to regenerate all SDK artifacts (stable stubs now exist alongside beta stubs)
• For each language (Go, C#, Rust, Unity): add stable SDK implementations; mark the Beta SDK methods as deprecated in the beta files
• Update [Stage:Beta] / [FeatureFlag:CountsAndLists] annotations in proto and SDK files to reflect stable status
• Open a follow-up issue to remove the beta.proto definitions and beta implementations ~5 releases after this graduation

Config & docs

• pkg/util/runtime/features.go — Move FeatureCountsAndLists to // Stable features (keep in featureDefaults as true)
• install/helm/agones/defaultfeaturegates.yaml — Move to Stable section
• build/Makefile — Remove from BETA_FEATURE_GATES
• cloudbuild.yaml — Remove CountsAndLists=false from the inverted e2e config
• site/content/en/docs/Guides/feature-stages.md — Move row from Beta → Stable table
• site/content/en/docs/Guides/counters-and-lists.md — Remove Beta stage references
• site/content/en/docs/Installation/upgrading.md — Fill in the "Stable" column for GetCounter, UpdateCounter, GetList, UpdateList, AddListValue, RemoveListValue in the SDK API stability table with the graduation release version
• test/upgrade/versionMap.yaml — Add stable-gates entry for this release

Tests

• Remove FeatureCountsAndLists skip guards from e2e tests: TestCounters, TestLists, TestCounterAndListGameServerAllocation, TestFleetAggregatedCounterStatus, TestFleetAggregatedListStatus, and all Counter/List autoscaler tests in test/e2e/
• Remove unit test feature-gate guards in pkg/gameserversets/, pkg/fleetautoscalers/, pkg/gameserverallocations/

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Suggested PR breakdown

Given the size of this work, splitting across multiple PRs is strongly recommended:

1. PR 1 — API validation + tests: Remove gates from the five validation webhook files and their unit tests
2. PR 2 — Controllers, allocation, autoscaler, metrics, sorting + tests: All the controller and allocation logic gate removal
3. PR 3 — SDK server gate removal: Remove FeatureCountsAndLists checks from pkg/sdkserver/sdkserver.go and pkg/sdkserver/localsdk.go
4. PR 4 — SDK proto + codegen: Copy Counter/List RPC definitions from beta.proto into sdk.proto (leaving beta.proto intact); run make gen-all-sdk-grpc so stable stubs now exist alongside beta stubs — must land before per-language PRs
5. PR 5 (Go), PR 6 (C#), PR 7 (Rust), PR 8 (Unity) — Per-language SDK promotion: Add stable implementations; mark beta methods as deprecated in each language's beta file (do not delete yet — removal comes ~5 releases later via a follow-up issue)
6. PR 9 — Config & docs: features.go, Helm, Makefile, cloudbuild, feature-stages, counters-and-lists guide, upgrading.md SDK table, versionMap (can land any time after PRs 1–3 are merged)

Verification

• make lint test-go passes with no CountsAndLists-gated skips
• E2E tests TestCounters, TestLists, and all Counter/List allocation/autoscaler tests run without skip guards
• A GameServer with counters/lists defined can be created without setting the feature gate
• Setting CountsAndLists=true or CountsAndLists=false in the feature gate string must not cause a parse error (covered by keeping the const in featureDefaults)
• make gen-all-sdk-grpc produces no unexpected diff after the proto promotion