Fix typing of Self

The method always returned a `ComponentMetricFetcher` (or
`LatestMetricsFetcher`), even for subclasses, which is not what `Self`
is. To make sure that an instance of the subclass is returned we just
use `cls` to call `__new__` instead of `ComponentMetricFetcher` (and fix
the type hint for `LatestMetricsFetcher`).

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/sdk/timeseries/battery_pool/_component_metric_fetcher.py

@@ -57,7 +57,7 @@ async def async_new(
         Returns:
             This class instance.
         """
-        self: ComponentMetricFetcher = ComponentMetricFetcher.__new__(cls)
+        self: Self = cls.__new__(cls)
         self._component_id = component_id
         self._metrics = metrics
         return self
@@ -94,7 +94,7 @@ async def async_new(
         Returns:
             This class instance
         """
-        self: LatestMetricsFetcher[T] = await super().async_new(component_id, metrics)
+        self: Self = await super().async_new(component_id, metrics)
 
         for metric in metrics:
             # pylint: disable=protected-access