Debug trace logger should no-op if application manually adds the ApplicationTraceLoggerProvider itself

[DamianEdwards]

Currently, an application that wishes to use the ApplicationInsightsLoggerProvider to redirect log messages to Application Insights trace telemetry needs to disable the debug logger (via ApplicationInsightsOptions.EnableDebugLogger) or they'll end up with duplicate trace messages while debugging the application.

The logger added by ApplicationInsightsOptions.EnableDebugLogger should coordinate this itself to avoid duplicate messages rather than the application having to do it manually (maybe by using a marker TelemetryInitializer to detect when another logger instance has been added and in that case evaluate that instance's configured filter [stored on the marker] and no-op if the filter already applies).