fix: add prefix to standalone metrics - #27526

Documentation Check

Updates Needed

• docs/ai-coder/ai-gateway/monitoring.md - Standalone AI Gateway replicas (coder ai-gateway start) previously exported unprefixed series (interceptions_total, key_pool_state, provider reload metrics) and now export them as coder_ai_gateway_*. Add a note that standalone replica series names change on upgrade, and that unlike the embedded gateway in coderd, standalone replicas emit only the new prefix with no legacy coder_aibridged_* alias.
• docs/ai-coder/ai-gateway/rebranding-migration.md - The Metrics section states that "Both the old and new metric names are emitted simultaneously today" and that "Every series is exported under both prefixes". That only holds for the gateway embedded in coderd, which wraps its registry with NewMetricAliasRegisterer. Scope the claim to the embedded gateway and state that standalone replicas expose coder_ai_gateway_* only.

Everything else in the diff (extracting the prefix into aibridgemetrics.PrometheusMetricPrefix) is internal and needs no docs. The metric names listed in docs/admin/integrations/prometheus.md are unaffected, since that page documents the embedded gateway.

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