Publish "client_count" topic

[jhurliman]

Taken from rosbridge:

# Publisher for number of connected clients
# QoS profile with transient local durability (latched topic in ROS 1).
client_count_qos_profile = QoSProfile(
    depth=10,
    durability=DurabilityPolicy.TRANSIENT_LOCAL,
)

RosbridgeWebSocket.client_count_pub = self.create_publisher(
    Int32, "client_count", qos_profile=client_count_qos_profile
)
RosbridgeWebSocket.client_count_pub.publish(Int32(data=0))

[achim-k]

The main motivation behind the client_count topic was to let a client see that they might not be the only one that is making changes to the server configuration (e.g. through setting parameters or calling other kind of ros services).

Having this is useful, but I wonder if we should not add it directly to the ws-protocol spec instead of having it a separate ROS topic

[jhurliman]

One side effect of publishing a ROS topic is that /client_count can end up in a recording. I'm not sure if that is useful though, it does seem like something only WebSocket-connected clients would care about.

To support this in ws-protocol, we would need something like the status op that servers can push to clients at will, but with more structure perhaps like a key/value pair and using client_count as the key?

[achim-k]

To support this in ws-protocol, we would need something like the status op that servers can push to clients at will, but with more structure perhaps like a key/value pair and using client_count as the key?

Or maybe we just add client_count as an optional field to the status message?