You manage applications deployed on Google Kubernetes Engine (GKE) clusters across multiple Google Cloud projects.

Comprehensive and Detailed 150 to 200 words of Explanation From Google Cloud DevOps guides documents:

Google Cloud’s recommended practice for scalable, multi-cluster monitoring is Google Cloud Managed Service for Prometheus (GMP). This service is a fully managed, multi-cloud-capable solution built on top of Monarch (the same globally scalable time-series database Google uses internally). By using managed collection, GKE clusters automatically deploy collectors that scrape metrics without the operational overhead of managing a manual Prometheus server, scaling, or long-term storage (sharding).

This solution satisfies the "centralized and scalable" requirement because GMP allows you to query data across multiple projects and clusters using PromQL, which provides the flexible query language requested. While the Ops Agent (Option C) is useful for VM-based workloads, GMP is the purpose-built solution for Kubernetes environments. Standard sidecar deployments (Option A) or self-managed admin clusters (Option D) introduce significant administrative toil and fail to leverage the global scale and high availability of the Cloud Monitoring backend. By centralizing metrics in Metrics Explorer, SRE teams gain a unified view of system health, making it easier to define SLOs and manage incidents across a complex microservices landscape.