[Metrics] Add audio SLOs + cross-stage transfer families + per-(stage, replica) wrap for upstream vllm:* (#3576)

Signed-off-by: vraiti <vraiti@redhat.com>
Signed-off-by: LHXuuu <xulianhao.xlh@antgroup.com>
Co-authored-by: vraiti <vraiti@redhat.com>

docs/design/index.md

@@ -13,6 +13,10 @@ This section contains design documents and architecture specifications for vLLM-
 - [Adding Step Execution Support for Diffusion Pipelines](feature/diffusion_step_execution.md)
 - [Continuous Batching for Step-Wise Diffusion](feature/diffusion_continuous_batching.md)
 
+## Infrastructure Design Documents
+
+- [Prometheus Metrics](metrics.md)
+
 ## Module Design Documents
 
 - [AR Module](module/ar_module.md)