Support clearing the latest value in the LatestValueCache

This can be the overall latest value or the latest value for a
specific key.  This can be used to clean up old keys for which no
further messages are expected.

Signed-off-by: Sahas Subramanian <sahas.subramanian@proton.me>

Author: shsms

src/frequenz/channels/_latest_value_cache.py

@@ -194,6 +194,22 @@ def has_value(self, key: HashableT | Sentinel = NO_KEY) -> bool:
             return key in self._latest_value_by_key
         return not isinstance(self._latest_value, Sentinel)
 
+    def clear(self, key: HashableT | Sentinel = NO_KEY) -> None:
+        """Clear the latest value or the latest value for a specific key.
+
+        If `key` is provided, it clears the latest value for that key. If no key is
+        provided, it clears the latest value received overall.
+
+        Args:
+            key: An optional key to clear the latest value for that key. If not
+                provided, it clears the latest value received overall.
+        """
+        if not isinstance(key, Sentinel):
+            _ = self._latest_value_by_key.pop(key, None)
+            return
+
+        self._latest_value = NO_VALUE_RECEIVED
+
     async def _run(self) -> None:
         async for value in self._receiver:
             self._latest_value = value