Adds /api/v1/admin/server-metrics/{catalog,instances,query} so SaaS control
planes can build the server-health dashboard without direct ClickHouse
access. One generic /query endpoint covers every panel in the
server-self-metrics doc: aggregation (avg/sum/max/min/latest), group-by-tag,
filter-by-tag, counter-delta mode with per-server_instance_id rotation
handling, and a derived 'mean' statistic for timers. Regex-validated
identifiers, parameterised literals, 31-day range cap, 500-series response
cap. ADMIN-only via the existing /api/v1/admin/** RBAC gate. Docs updated:
all 17 suggested panels now expressed as single-endpoint queries.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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