Migrate docs from Sphinx to MkDocs (#18145)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/serving/metrics.md

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+# 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]:
+
+```console
+vllm serve unsloth/Llama-3.2-1B-Instruct
+```
+
+Then query the endpoint to get the latest metrics from the server:
+
+```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:
+
+```python
+--8<-- "vllm/engine/metrics.py:metrics-definitions"
+```
+
+The following metrics are deprecated and due to be removed in a future version:
+
+- `vllm:num_requests_swapped`, `vllm:cpu_cache_usage_perc`, and
+  `vllm:cpu_prefix_cache_hit_rate` because KV cache offloading is not
+  used in V1.
+- `vllm:gpu_prefix_cache_hit_rate` is replaced by queries+hits
+  counters in V1.
+- `vllm:time_in_queue_requests` because it duplicates
+  `vllm:request_queue_time_seconds`.
+- `vllm:model_forward_time_milliseconds` and
+  `vllm:model_execute_time_milliseconds` because
+  prefill/decode/inference time metrics should be used instead.
+
+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`.