[FEATURE] Worker log-format setting replaces GCE detection #3673

The gVisor and micro-VM workers call metadata.OnGCE() to choose the actor log label key. Replace the detection with an explicit setting the controller propagates to workers.

Tracks "Replace GCE detection with a log-format setting" in the Provider-Neutral Core workstream of the Extensible Install Roadmap.

Current state (0b635b80): cmd/ateworker-gvisor/main.go, cmd/ateworker-microvm/main.go and internal/actorlog/logger.go call metadata.OnGCE().

Required changes (two steps)

1. Add a worker log-format setting (Proposed: ATE_WORKER_LOG_FORMAT in ate-otel-config, values text|json, and add it to [FEATURE] Establish cluster-side install setting records #3172). Its default is today's automatic detection. The controller propagates it to workers.
2. After the GKE parity release (GKE parity release: install through the GKE Component ai-on-gke/substrate-gke#151): default to a neutral format and remove metadata.OnGCE() and the cloud.google.com/go/compute/metadata dependency. The GKE Component sets GKE's format (Complete the GKE kustomize Component ai-on-gke/substrate-gke#152).

Dependencies: step 2 waits for ai-on-gke/substrate-gke#151.

Breaking change: step 2 changes the default log format on GCE.

Enforcement

• Tests for each format.
• No compute/metadata import in the workers after step 2.

Parent Issue: #3161