ref(metrics): Update type hints for tag values

Remove None, Tuple, and List from the MetricTagValue type as these are not supported and might confuse the user

sentry_sdk/_types.py

@@ -173,14 +173,7 @@
     MetricTagsInternal = Tuple[Tuple[str, str], ...]
 
     # External representation of tags as a dictionary.
-    MetricTagValue = Union[
-        str,
-        int,
-        float,
-        None,
-        List[Union[int, str, float, None]],
-        Tuple[Union[int, str, float, None], ...],
-    ]
+    MetricTagValue = Union[str, int, float]
     MetricTags = Mapping[str, MetricTagValue]
 
     # Value inside the generator for the metric value.