[Azure.Core] Logging Sample Enhancement (Azure#25389)

# Summary

The focus of these changes is to enhance the logging section of the diagnostics sample to include details about the lifetime of the `AzureEventSourceListener`.  We received some customer feedback that it was unclear where they needed to create the listener and that it had to have the appropriate scope to be able to capture logs.

Author: jsquire

sdk/core/Azure.Core/samples/Diagnostics.md

@@ -9,13 +9,17 @@ Azure SDKs produce various log messages that include information about:
 2. Authentication attempts
 3. Retries
 
-The simplest way to see the logs is to enable the console logging.
+The simplest way to see the logs is to enable the console logging using the [`AzureEventSourceListener`](https://docs.microsoft.com/dotnet/api/azure.core.diagnostics.azureeventsourcelistener?view=azure-dotnet).
 
 ```C# Snippet:ConsoleLogging
 // Setup a listener to monitor logged events.
 using AzureEventSourceListener listener = AzureEventSourceListener.CreateConsoleLogger();
 ```
 
+### Azure Event Source Listener lifetime
+
+In order for the `AzureEventSourceListener` to collect logs, it must be in scope and active while the client library is in use.  If the listener is disposed or otherwise out of scope, logs cannot be collected.  Generally, we recommend creating the listener as a top-level member of the class where the Event Hubs client being inspected is used.
+
 ### Enabling content logging
 
 By default only URI and headers are logged. To enable content logging, set the `Diagnostics.IsLoggingContentEnabled` client option: