fixup! Add a wall clock attached timer for the resampler

Add comment about using __setattr__ to bypass frozen dataclass
restriction.

Author: llucax

src/frequenz/sdk/timeseries/_resampling/_wall_clock_timer.py

@@ -248,6 +248,9 @@ def __post_init__(self) -> None:
                     wall_clock_elapsed,
                 )
                 wall_clock_elapsed = self.monotonic_elapsed * 0.1
+            # We need to use __setattr__ here to bypass the frozen nature of the
+            # dataclass. Since we are constructing the class, this is fine and the only
+            # way to set calculated defaults in frozen dataclasses at the moment.
             super().__setattr__(
                 "wall_clock_factor", self.monotonic_elapsed / wall_clock_elapsed
             )