Track sender count on sender create and close

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

Author: shsms

src/frequenz/channels/_broadcast.py

@@ -207,6 +207,9 @@ def __init__(self, *, name: str, resend_latest: bool = False) -> None:
         self._recv_cv: Condition = Condition()
         """The condition to wait for data in the channel's buffer."""
 
+        self._sender_count: int = 0
+        """The number of senders attached to this channel."""
+
         self._receivers: dict[
             int, weakref.ReferenceType[_Receiver[ChannelMessageT]]
         ] = {}
@@ -337,6 +340,8 @@ def __init__(self, channel: Broadcast[_T], /) -> None:
         self._closed: bool = False
         """Whether this sender is closed."""
 
+        self._channel._sender_count += 1
+
     @override
     async def send(self, message: _T, /) -> None:
         """Send a message to all broadcast receivers.
@@ -380,6 +385,7 @@ def close(self) -> None:
         [SenderError][frequenz.channels.SenderError].
         """
         self._closed = True
+        self._channel._sender_count -= 1
 
     @override
     def clone(self) -> _Sender[_T]: