Add count_valid method to ring buffer

The current len magic returns the count of samples in the ring buffer
that are determined valid. This definition is not unambiguously obvious
to the caller. The current logic is therefore moved to its own more
descriptive method which in the first step is called by the len magic.
This prepares removal of the len magic in subsequent changes.

Signed-off-by: cwasicki <[email protected]>

Author: cwasicki

src/frequenz/sdk/timeseries/_ringbuffer/buffer.py

@@ -594,11 +594,11 @@ def __getitem__(self, index_or_slice: SupportsIndex | slice) -> float | FloatArr
         """
         return self._buffer.__getitem__(index_or_slice)
 
-    def __len__(self) -> int:
-        """Return the amount of items that this container currently holds.
+    def count_valid(self) -> int:
+        """Count the number of valid items that this buffer currently holds.
 
         Returns:
-            The length.
+            The number of valid items in this buffer.
         """
         if self._datetime_newest == self._DATETIME_MIN:
             return 0
@@ -624,3 +624,7 @@ def __len__(self) -> int:
             return len(self._buffer) - start_index + end_index + 1 - sum_missing_entries
 
         return end_index + 1 - start_index - sum_missing_entries
+
+    def __len__(self) -> int:
+        """Return the count of valid items that this container currently holds."""
+        return self.count_valid()