docs: clarify that /metrics endpoint requires separate authentication

[pigeio]

What is this change?

This PR adds a security clarification note to the RBAC documentation explaining that the /metrics and /health HTTP endpoints are not protected by V3 RBAC authentication (auth enable).

Why is this needed?

Operators may assume that enabling authentication with etcdctl auth enable protects all endpoints. However, the /metrics endpoint uses a separate HTTP handler and requires mTLS or network isolation for protection.

Code references:

• V3 auth layer: server/etcdserver/api/v3rpc/interceptor.go - guards gRPC API
• Metrics handler: server/embed/etcd.go - separate HTTP handler

Changes

Added a new section "Security Scope of Authentication" after the "Enabling authentication" section in rbac.md that:

• Clarifies what V3 auth protects (gRPC API operations)
• Adds a warning note about /metrics and /health endpoints
• Provides mitigation options (mTLS, --listen-metrics-urls, network policies)

Related

• Analysis: https://gist.github.com/pigeio/2f2c7dfe387374434a72dbb2e4b40ff0

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[k8s-ci-robot]

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