Improve journaling with clients telemetry

[dborovcanin]

Is your feature request related to a problem? Please describe.

I want the ability to see some basic telemetry related to my domain.

Describe the feature you are requesting, as well as the possible use case(s) for it.

Here is an example of telemetry I want to check:

• number of connections per client
• number of subscriptions per client
• number of inbound and outbound messages per client
• total number of all the above per domain
• last seen per client

We will use Journal service, but not as a consumer/collector, but as an aggregator and API provider for this information, and have a background process to sync with the actual collector.

This is an umbrella issue we'll handle in the following way:

• Add API and Repository implementation for the Client stats #2648
• Do research on telemetry collectors and how we can use them, and if we are missing to send some events #2649

Indicate the importance of this feature to you.

Must-have

Anything else?

No response

[felixgateru]

NATS monitors our subscriptions differently for the adapters. For COAP individual subscriptions are made to NATS but for MQTT a single susbcription for all the subjects is made. This means we will not be able to directly get the number of active subscriptions from NATS.