Add documentation for new APPLICATIONINSIGHTS_CONNECTIONSTRING app setting

Author: gzuber

articles/azure-functions/functions-app-settings.md

@@ -15,12 +15,20 @@ There are other global configuration options in the [host.json](functions-host-j
 
 ## APPINSIGHTS_INSTRUMENTATIONKEY
 
-The Application Insights instrumentation key if you're using Application Insights. See [Monitor Azure Functions](functions-monitoring.md).
+The Application Insights instrumentation key if you're using Application Insights. See [Monitor Azure Functions](functions-monitoring.md). Only one of `APPINSIGHTS_INSTRUMENTATIONKEY` or `APPLICATIONINSIGHTS_CONNECTIONSTRING` is recommended.
 
 |Key|Sample value|
 |---|------------|
 |APPINSIGHTS_INSTRUMENTATIONKEY|5dbdd5e9-af77-484b-9032-64f83bb83bb|
 
+## APPLICATIONINSIGHTS_CONNECTIONSTRING
+
+The Application Insights connection string if you're using Application Insights and require the added customizability that connection strings provide over instrumentation keys. See [Connection strings](/articles/azure-monitor/app/sdk-connection-string.md) for more details. Only one of `APPINSIGHTS_INSTRUMENTATIONKEY` or `APPLICATIONINSIGHTS_CONNECTIONSTRING` is recommended.
+
+|Key|Sample value|
+|---|------------|
+|APPLICATIONINSIGHTS_CONNECTIONSTRING|InstrumentationKey=5dbdd5e9-af77-484b-9032-64f83bb83bb;IngestionEndpoint=https://custom.com:111/;LiveEndpoint=https://custom.com:222/;ProfilerEndpoint=https://custom.com:333/;SnapshotEndpoint=https://custom.com:444/;|
+
 ## AZURE_FUNCTIONS_ENVIRONMENT
 
 In version 2.x and later versions of the Functions runtime, configures app behavior based on the runtime environment. This value is [read during initialization](https://github.com/Azure/azure-functions-host/blob/dev/src/WebJobs.Script.WebHost/Program.cs#L43). You can set `AZURE_FUNCTIONS_ENVIRONMENT` to any value, but [three values](/dotnet/api/microsoft.aspnetcore.hosting.environmentname) are supported: [Development](/dotnet/api/microsoft.aspnetcore.hosting.environmentname.development), [Staging](/dotnet/api/microsoft.aspnetcore.hosting.environmentname.staging), and [Production](/dotnet/api/microsoft.aspnetcore.hosting.environmentname.production). When `AZURE_FUNCTIONS_ENVIRONMENT` isn't set,  it defaults to `Development` on a local environment and `Production` on Azure. This setting should be used instead of `ASPNETCORE_ENVIRONMENT` to set the runtime environment.