fixup! Implement GetMicrogridMetadata

Add tests for `Lifetime`.

Author: llucax

src/frequenz/client/microgrid/_lifetime.py

@@ -45,12 +45,11 @@ def active_at(self, timestamp: datetime) -> bool:
         Returns:
             True if active at the given timestamp, False otherwise.
         """
-        # Handle start time - it's not active if start is in the future
         if self.start is not None and self.start > timestamp:
             return False
-        # Handle end time - active up to and including end time
         if self.end is not None:
             return self.end >= timestamp
+        # Both are None, so it is always active
         return True
 
     @cached_property