[CI/Build] mypy: Resolve some errors from checking vllm/engine (#9267)

Signed-off-by: Russell Bryant <rbryant@redhat.com>

vllm/engine/metrics.py

@@ -1,6 +1,6 @@
 from typing import TYPE_CHECKING
 from typing import Counter as CollectionsCounter
-from typing import Dict, List, Optional, Union
+from typing import Dict, List, Optional, Type, Union, cast
 
 import numpy as np
 import prometheus_client
@@ -249,10 +249,11 @@ class _RayHistogramWrapper:
                  labelnames: Optional[List[str]] = None,
                  buckets: Optional[List[float]] = None):
         labelnames_tuple = tuple(labelnames) if labelnames else None
+        boundaries = buckets if buckets else []
         self._histogram = ray_metrics.Histogram(name=name,
                                                 description=documentation,
                                                 tag_keys=labelnames_tuple,
-                                                boundaries=buckets)
+                                                boundaries=boundaries)
 
     def labels(self, **labels):
         self._histogram.set_default_tags(labels)
@@ -267,9 +268,12 @@ class RayMetrics(Metrics):
     RayMetrics is used by RayPrometheusStatLogger to log to Ray metrics.
     Provides the same metrics as Metrics but uses Ray's util.metrics library.
     """
-    _gauge_cls = _RayGaugeWrapper
-    _counter_cls = _RayCounterWrapper
-    _histogram_cls = _RayHistogramWrapper
+    _gauge_cls: Type[prometheus_client.Gauge] = cast(
+        Type[prometheus_client.Gauge], _RayGaugeWrapper)
+    _counter_cls: Type[prometheus_client.Counter] = cast(
+        Type[prometheus_client.Counter], _RayCounterWrapper)
+    _histogram_cls: Type[prometheus_client.Histogram] = cast(
+        Type[prometheus_client.Histogram], _RayHistogramWrapper)
 
     def __init__(self, labelnames: List[str], max_model_len: int):
         if ray_metrics is None: