ApplicationInsightsPublisher: TelemetryClient from DI

[espenrl]

Hi,

The ApplicationInsightsPublisher instantiates its own TelemetryClient. According to Microsoft documentation the best practice is to get it using the one in the DI container. And then it could me made a readonly field instead of static field with a lock.

https://github.com/Microsoft/ApplicationInsights-aspnetcore/wiki/Custom-Configuration

AspNetCore.Diagnostics.HealthChecks/src/HealthChecks.Publisher.ApplicationInsights/ApplicationInsightsPublisher.cs

Lines 22 to 23 in 2d867c9

	private static TelemetryClient _client;
	private static readonly object sync_root = new object();

AspNetCore.Diagnostics.HealthChecks/src/HealthChecks.Publisher.ApplicationInsights/ApplicationInsightsPublisher.cs

Lines 85 to 105 in 2d867c9

	private TelemetryClient GetOrCreateTelemetryClient()
	{
	if (_client == null)
	{
	lock (sync_root)
	{
	if (_client == null)
	{
	//override instrumentation key or use default instrumentation
	//key active on the project.
	var configuration = string.IsNullOrWhiteSpace(_instrumentationKey)
	? TelemetryConfiguration.Active
	: new TelemetryConfiguration(_instrumentationKey);
	_client = new TelemetryClient(configuration);
	}
	}
	}
	return _client;
	}

[unaizorrilla]

Can you send a PR with this change?

[CarlosLanderas]

Hello @espenrl, I was preparing a PR for this but after talking with @unaizorrilla this behaviour is on purpose.

Users might want to use default telemetry client for application monitoring and a different client for health checks monitoring. If you don't provide a telemetry key the health check will use the active telemetry configuration.