Make sure DiagnosticSubscribers can be registered only once

[gregkalapos]

From #1091 (comment).

#1091 makes sure that AspNetCoreErrorDiagnosticsSubscriber can only be registered once.

That's the only check - other DiagnosticsSubscribers can be registered multiple times. Basically all DiagnosticsSubscribers are designed to be registered only once - if we register them multiple times each instance will create its own spans, which never really makes sense.

For example:

//adds the same HttpDiagnosticsSubscriber twice
app.UseElasticApm(_agent, _agent.Logger, new HttpDiagnosticsSubscriber(), new HttpDiagnosticsSubscriber());

//this could be called multiple times and also does not care if HttpDiagnosticsSubscriber is already added
Agent.Subscribe(new HttpDiagnosticsSubscriber());

The task here would be to make sure that each these subscribers can only be registered once.

First idea: let's just check if an instance is already added, if so, then ignore each subsequent registration for an already registered listener.