docs: document NIXL Prometheus telemetry metrics in observability guide (#7141)

Signed-off-by: Keiven Chang <keivenchang@users.noreply.github.com>
Co-authored-by: Keiven Chang <keivenchang@users.noreply.github.com>

docs/observability/metrics.md

@@ -18,6 +18,7 @@ Dynamo provides built-in metrics capabilities through the Dynamo metrics API, wh
 |----------|-------------|---------|---------|
 | `DYN_SYSTEM_PORT` | Backend component metrics/health port | `-1` (disabled) | `8081` |
 | `DYN_HTTP_PORT` | Frontend HTTP port (also configurable via `--http-port` flag) | `8000` | `8000` |
+| `NIXL_TELEMETRY_ENABLE` | Enable NIXL telemetry (see [NIXL Telemetry Metrics](#nixl-telemetry-metrics)). Options: `y`, `n` | `n` (disabled) | `y` |
 
 ## Getting Started Quickly
 
@@ -277,6 +278,29 @@ These appear once workers register and begin serving requests. They are register
 
 In disaggregated mode, the `worker_type` label shows both `"prefill"` and `"decode"` values; in aggregated mode, all workers report as `"decode"`.
 
+## NIXL Telemetry Metrics
+
+[NIXL](https://github.com/ai-dynamo/nixl) exposes its own Prometheus metrics on a **separate port** from Dynamo metrics. These metrics track KV cache and embedding data transfers and are only populated during **disaggregated serving** or **multimodal embedding transfers**.
+
+To enable, set these environment variables on your worker process:
+
+```bash
+# Prefill worker
+NIXL_TELEMETRY_ENABLE=y NIXL_TELEMETRY_EXPORTER=prometheus \
+  NIXL_TELEMETRY_PROMETHEUS_PORT=19090 DYN_SYSTEM_PORT=8081 \
+  python -m dynamo.vllm --model <model> --disaggregation-mode prefill
+
+# Decode worker (different NIXL port to avoid collision)
+NIXL_TELEMETRY_ENABLE=y NIXL_TELEMETRY_EXPORTER=prometheus \
+  NIXL_TELEMETRY_PROMETHEUS_PORT=19091 DYN_SYSTEM_PORT=8082 \
+  python -m dynamo.vllm --model <model> --disaggregation-mode decode
+
+# Scrape NIXL metrics (separate from Dynamo metrics on 8081/8082)
+curl http://localhost:19090/metrics
+```
+
+For the full list of metrics, configuration options, and architecture details, see the upstream [NIXL Telemetry documentation](https://github.com/ai-dynamo/nixl/blob/main/docs/telemetry.md) and [Prometheus exporter README](https://github.com/ai-dynamo/nixl/blob/main/src/plugins/telemetry/prometheus/README.md). For Kubernetes, see [Enable NIXL Telemetry](../kubernetes/observability/metrics.md#enable-nixl-telemetry-optional).
+
 ## Related Documentation
 
 - [Distributed Runtime Architecture](../design-docs/distributed-runtime.md)