Multi-tenant observability platform

Design a multi-tenant SaaS observability platform shared by 5,000 customer organizations, ranging from tiny startups to a few whales sending 100x the median traffic. All tenants share the same ingest, storage, and query clusters. One noisy tenant must never degrade ingest or query latency for others, data must be strictly isolated, and per-tenant usage must be metered for billing. Design tenant isolation across the write path, storage, and query path, plus quota enforcement.