Add vLLM v0.18.1 source tree with KV transfer abort fix

third_party/vllm/ now tracked in git for direct patch management.
Based on vLLM v0.18.1 release with one patch applied:

  vllm/v1/core/sched/scheduler.py:
    Replace fatal assert with graceful skip when KV transfer callback
    arrives for an already-aborted request during PD disaggregated serving.

Future vLLM modifications should be made directly in third_party/vllm/
and committed normally. The patches/ directory is kept as documentation
of what changed from upstream.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

third_party/vllm/docs/usage/metrics.md

@@ -0,0 +1,58 @@
+# Production Metrics
+
+vLLM exposes a number of metrics that can be used to monitor the health of the
+system. These metrics are exposed via the `/metrics` endpoint on the vLLM
+OpenAI compatible API server.
+
+You can start the server using Python, or using [Docker](../deployment/docker.md):
+
+```bash
+vllm serve unsloth/Llama-3.2-1B-Instruct
+```
+
+Then query the endpoint to get the latest metrics from the server:
+
+??? console "Output"
+
+    ```console
+    $ curl http://0.0.0.0:8000/metrics
+
+    # HELP vllm:iteration_tokens_total Histogram of number of tokens per engine_step.
+    # TYPE vllm:iteration_tokens_total histogram
+    vllm:iteration_tokens_total_sum{model_name="unsloth/Llama-3.2-1B-Instruct"} 0.0
+    vllm:iteration_tokens_total_bucket{le="1.0",model_name="unsloth/Llama-3.2-1B-Instruct"} 3.0
+    vllm:iteration_tokens_total_bucket{le="8.0",model_name="unsloth/Llama-3.2-1B-Instruct"} 3.0
+    vllm:iteration_tokens_total_bucket{le="16.0",model_name="unsloth/Llama-3.2-1B-Instruct"} 3.0
+    vllm:iteration_tokens_total_bucket{le="32.0",model_name="unsloth/Llama-3.2-1B-Instruct"} 3.0
+    vllm:iteration_tokens_total_bucket{le="64.0",model_name="unsloth/Llama-3.2-1B-Instruct"} 3.0
+    vllm:iteration_tokens_total_bucket{le="128.0",model_name="unsloth/Llama-3.2-1B-Instruct"} 3.0
+    vllm:iteration_tokens_total_bucket{le="256.0",model_name="unsloth/Llama-3.2-1B-Instruct"} 3.0
+    vllm:iteration_tokens_total_bucket{le="512.0",model_name="unsloth/Llama-3.2-1B-Instruct"} 3.0
+    ...
+    ```
+
+The following metrics are exposed:
+
+## General Metrics
+
+--8<-- "docs/generated/metrics/general.inc.md"
+
+## Speculative Decoding Metrics
+
+--8<-- "docs/generated/metrics/spec_decode.inc.md"
+
+## NIXL KV Connector Metrics
+
+--8<-- "docs/generated/metrics/nixl_connector.inc.md"
+
+## Model Flops Utilization (MFU) Performance Metrics
+
+These metrics are available via `--enable-mfu-metrics`:
+
+--8<-- "docs/generated/metrics/perf.inc.md"
+
+## Deprecation Policy
+
+Note: when metrics are deprecated in version `X.Y`, they are hidden in version `X.Y+1`
+but can be re-enabled using the `--show-hidden-metrics-for-version=X.Y` escape hatch,
+and are then removed in version `X.Y+2`.