Add Client Count Topic to Foxlgove_bridge

[ndmmdn]

Changelog

• Foxglove_bridge now publishes the number of foxglove clients connected to it under /foxglove_bridge/client_count

Docs

• I added a mention of the topic to the foxglove_bridge readme

Description

• There are several examples where it is useful to no if a client is connected to the foxglove bridge, for example to pause other bandwidth intensive processes. Additionally, similar to the who command in bash, it can also be useful to know if there are multiple people connected to the same robot. Both of these cases can now be detected via the new client count topic.

there exists the test_callbacks test to allow testing on the c++ server level.

Additionally, the new functionality was tested by build and running the foxglove_bridge as described in the readme. In a separate terminal, the /foxglove_bridge/client_count topic was subscribed to. Then using several foxglove instances on the same laptop, connections and disconnections were made to verify the client count is updated correctly.

Before	After

This is a refactor/adaptation of this foxglove_bridge PR