Support for System Topic ($SYS)

[ssayebee]

I am trying to configure a dashboard using the $SYS/# topic.

Although there is support for special characters such as # and +, the $ character is still not supported, preventing me from subscribing to the $SYS/# topic.

https://github.com/grafana/mqtt-datasource/blob/ef46f0a216c336719aaa6039a87192855c098514/src/datasource.ts#L13-L14

I do not fully understand the source code, but I am aware that special character support for + and # is provided using the following trick:

+ -> __PLUS__
# -> __HASH__

I would like to request support for the $ character as well using __DOLLAR__.

Thank you for your efforts.