Support for modern telemetry beyond primitive logging

### Describe the feature request

Please also see https://github.com/dotnet/MQTTnet/discussions/1625, https://github.com/dotnet/MQTTnet/pull/1866, and https://github.com/dotnet/MQTTnet/pull/1867.

* The IMqttNetLogger interface is inadequate for supporting telemetry in a large-scale consumer-grade context, as the logger.Publish method only provides telemetry for events (moments in time). It does not capture activities that span an interval in time.
* Current server properties leave gaps for meaningful metrics such as the number of active connections.
* The current server event hooks support extensibility and custom application behavior but leave gaps for capturing metrics such as connection and delivery counts.

### Which project is your feature request related to?

- Server

### Describe the solution you'd like
* .NET already has a built-in abstraction for activities, System.Diagnostics.Activity. Use it directly (preferred) or provide interfaces that enable wrapping of ActivitySource.BeginActivity, Activity::Stop, Activity.Current, and Activity::AddEvent.
* Add equivalents to ActivitySource::BeginActivity and Activity::Stop at the beginning and end of significant MQTT activities, such as processing of messages.
* Augment or replace calls to IMqttNetLogger::Publish with equivalents to Activity.Current::AddEvent.
* Add server properties for meaningful gauges like current active connections.
* Add events that support meaningful counters like total connections.

### Describe alternatives you've considered
We are already doing these in our own MqttNet fork but would like to reduce how much we're drifting from the upstream fork.