Pipeline Stage Visibility Metrics (Limiter, Enforcer, Optimizer)

[ev-shindin]

Add metrics exposing how pipeline stages modify scaling decisions.

Metrics:

wva_decisions_limited_total         counter {variant_name, namespace, limiter_name}
wva_available_gpus                  gauge   {accelerator_type}
wva_enforcer_modifications_total    counter {policy_type}
wva_optimizer_active                gauge   {optimizer_name}

Implementation:

• Limiter: Source from VariantDecision.WasLimited, LimitedBy fields; TypeInventory.remaining in DefaultLimiter
• Enforcer: Increment when enforcer modifies a decision. Policy types: scale_to_zero, minimum_replicas, stabilization_window
• Optimizer: Emit 1 for active optimizer (cost_aware or greedy_by_saturation), 0 for inactive

Acceptance Criteria:

• Limiter constraint events tracked with variant + limiter name
• Available GPU gauge reflects current inventory
• Enforcer modification counter tracks each policy type
• Active optimizer gauge reflects current config
• Unit tests for each pipeline stage metric