Add MqttCount metric to STATE (#6155)

[s-hadinger]

Description:

Add MqttCount in State telemetry to avoid polling from TDM (discussed on Discord)

Related issue (if applicable): fixes #6155

Checklist:

• The pull request is done against the latest dev branch
• Only relevant files were touched
• Only one feature/fix was added per PR.
• The code change is tested and works on core 2.3.0, 2.4.2 and 2.5.2
• The code change pass travis tests. Your PR cannot be merged unless tests pass
• I accept the CLA.