Offer list of users who want to measure a specific channel

[BECATRUE]

Feature you want to implement

To offer operation info on a specific channel, I will implement to offer the list of users who want to measure the channel.

How the feature is implemented

The list of users who want to measure a specific channel will be offered through the WebSocket /ws/operation/<int:ch>/.
Currently, only whether the measurement of a specific channel is running is offered through the WebSocket, thus I will add the list of users to the content.