ref(metrics): Improve type hints for set metrics (#3048)

Remove MetricValue from the set metric's value type hint. MetricValue is a union that includes floatwhich is correct based on how other parts of the code uses MetricValue. However, as floats are intentionally floored for set metrics, the current type hint might be confusing for a user as it looks like floats would work as unique values in a set.

Fixes GH-3038

sentry_sdk/metrics.py

@@ -926,7 +926,7 @@ def distribution(
 
 def set(
     key,  # type: str
-    value,  # type: MetricValue
+    value,  # type: Union[int, str]
     unit="none",  # type: MeasurementUnit
     tags=None,  # type: Optional[MetricTags]
     timestamp=None,  # type: Optional[Union[float, datetime]]