feat(sglang): relay forward pass metrics to event plane

[ishandhanani]

Summary

• Extend DynamoSglangPublisher with init_fpm_relay() to subscribe to sglang's ForwardPassMetrics ZMQ PUB and relay to the event plane
• Map DYN_FORWARDPASS_METRIC_PORT env var to sglang's server_args.forward_pass_metrics_port in args.py
• Uses the same FpmEventRelay Rust binding as the vLLM integration (feat: ForwardPassMetrics dynamo event plane integration #7250)
• Handles DP attention topology (one relay per local DP rank)

Motivation

Companion to sgl-project/sglang#20569 which adds per-iteration ForwardPassMetrics emission from sglang's scheduler. This PR wires the Dynamo side to receive those metrics and forward them to the event plane for consumption by the planner.

Architecture

sglang scheduler (SchedulerMetricsMixin):
  _emit_forward_pass_metrics() -> _FpmPublisherThread -> ZMQ PUB (localhost)

Dynamo parent (DynamoSglangPublisher):
  init_fpm_relay() -> FpmEventRelay (ZMQ SUB) -> Event Plane (NATS)

Consumer (planner):
  FpmEventSubscriber -> decode() -> ForwardPassMetrics

Changes

• args.py: Map DYN_FORWARDPASS_METRIC_PORT env var to server_args.forward_pass_metrics_port before engine creation
• publisher.py: Add init_fpm_relay() method reading port from server_args, cleanup in cleanup(), call from setup_sgl_metrics()

Test plan

• Wire compatibility verified: sglang-encoded ForwardPassMetrics decoded by Dynamo's decoder
• Live server test: sglang with --forward-pass-metrics-port 20380 emits prefill/decode metrics per iteration
• Full stack test with event plane consumer (requires NATS/etcd)