feat: add Micrometer Prometheus metrics to server
Adds micrometer-registry-prometheus and exposes /api/v1/prometheus
endpoint (unauthenticated for scraping). ServerMetrics component
provides business metrics beyond default JVM/HTTP:
Gauges: agents by state, SSE connections, buffer depths (execution,
processor, log, metrics), accumulator pending exchanges.
Counters: ingestion drops (buffer_full, no_agent, no_identity),
agent transitions (went_stale, went_dead, recovered), deployment
outcomes (running, failed, degraded), auth failures (invalid_token,
revoked, oidc_rejected).
Timers: ClickHouse flush duration by type, deployment duration.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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