docs: reorganize module design documents

Signed-off-by: hsliuustc0106 <liuhongsheng4@huawei.com>

docs/design/module/observability.md

@@ -0,0 +1,46 @@
+---
+title: Observability
+kind: module
+status: draft
+owners:
+  - "@lishunyang12"
+primary_code_paths:
+  - vllm_omni/metrics/**
+related_code_paths:
+  - vllm_omni/engine/**
+  - vllm_omni/entrypoints/**
+depends_on:
+  - engine_orchestration.md
+validation_paths:
+  - tests/metrics/**
+upstream_refs:
+  - vllm.v1.metrics
+last_reviewed: 2026-07-16
+---
+
+# Observability
+
+Observability defines stable metrics, logs, and correlation fields for tracing a
+request across entrypoints, orchestration, stages, and workers.
+
+## Candidate invariants
+
+### OBS-INV-001: Request correlation survives stage boundaries
+
+**Rule:** Logs and metrics for the same request SHOULD carry a stable request
+identifier and stage identity.
+
+### OBS-INV-002: Metric meaning is stable
+
+**Rule:** A metric's unit, labels, aggregation, and lifecycle MUST be documented;
+changing any of them is a contract change.
+
+### OBS-INV-003: Labels are bounded
+
+**Rule:** Metrics MUST NOT use request IDs, prompts, outputs, or other unbounded
+values as labels.
+
+## Safe-change guide
+
+Test registration, label sets, units, lifecycle resets, and representative
+multi-stage emission.