[V1][Metrics] Fix docs build

```
docs/source/serving/engine_args.md:14: ERROR: Failed to import "_engine_args_parser" from "vllm.engine.arg_utils".
No module named 'prometheus_client'
```

Signed-off-by: Mark McLoughlin <markmc@redhat.com>

Author: markmc

vllm/entrypoints/llm.py

@@ -4,7 +4,8 @@
 import warnings
 from collections.abc import Sequence
 from contextlib import contextmanager
-from typing import Any, Callable, ClassVar, Optional, Union, cast, overload
+from typing import (TYPE_CHECKING, Any, Callable, ClassVar, Optional, Union,
+                    cast, overload)
 
 import cloudpickle
 import torch.nn as nn
@@ -46,7 +47,9 @@
 from vllm.usage.usage_lib import UsageContext
 from vllm.utils import (Counter, Device, deprecate_args, deprecate_kwargs,
                         is_list_of)
-from vllm.v1.metrics.reader import Metric
+
+if TYPE_CHECKING:
+    from vllm.v1.metrics.reader import Metric
 
 logger = init_logger(__name__)
 
@@ -1299,7 +1302,7 @@ def wake_up(self, tags: Optional[list[str]] = None):
         """
         self.llm_engine.wake_up(tags)
 
-    def get_metrics(self) -> list[Metric]:
+    def get_metrics(self) -> list["Metric"]:
         """Return a snapshot of aggregated metrics from Prometheus.
 
         Returns: