Add mqttCount to State telemetry

[meingraham]

Have you look for this feature in other issues and in the wiki?
Yes

Is your feature request related to a problem? Please describe.
Not a problem. An enhancement in order to be able to monitor the health of the device without adding traffic to poll Status 6. If monitoring many devices, polling traffic is a lot of unnecessary noise.

Describe the solution you'd like
Add mqttCount to State message much like LinkCount is in that message.

Describe alternatives you've considered
Polling

Additional context
Discussed as best alternative with @ascillato & @jziolkowski in Discord.