Connection String env var - base, web, nlog

BASE/src/Microsoft.ApplicationInsights/Extensibility/TelemetryConfiguration.cs

@@ -357,6 +357,11 @@ internal void SetCloudRole(string serviceName, string serviceInstanceId = null,
 
         internal FeatureMetricEmissionHelper InitializeFeatureReporter()
         {
+            if (string.IsNullOrEmpty(this.connectionString))
+            {
+                return this.FeatureReporter;
+            }
+
             var connectionString = Microsoft.ApplicationInsights.Internal.ConnectionString.Parse(this.ConnectionString);
             var ciKey = connectionString.GetNonRequired("InstrumentationKey");
             this.FeatureReporter = FeatureMetricEmissionHelper.GetOrCreate(ciKey, this.extensionVersion);
@@ -381,14 +386,27 @@ internal OpenTelemetrySdk Build()
                     return this.openTelemetrySdk;
                 }
 
+                // If no connection string was explicitly set, fall back to the environment variable.
+                if (string.IsNullOrEmpty(this.connectionString))
+                {
+                    var envConnectionString = Environment.GetEnvironmentVariable("APPLICATIONINSIGHTS_CONNECTION_STRING");
+                    if (!string.IsNullOrEmpty(envConnectionString))
+                    {
+                        this.connectionString = envConnectionString;
+                    }
+                }
+
                 Environment.SetEnvironmentVariable("OTEL_SDK_DISABLED", this.disableTelemetry ? "true" : "false");
 
                 this.openTelemetrySdk = OpenTelemetrySdk.Create(builder =>
                 {
                     this.builderConfiguration(builder);
                     builder.SetAzureMonitorExporter(options =>
                     {
-                        options.ConnectionString = this.connectionString;
+                        if (!string.IsNullOrEmpty(this.connectionString))
+                        {
+                            options.ConnectionString = this.connectionString;
+                        }
 
                         if (this.tracesPerSecond.HasValue)
                         {

LOGGING/src/NLogTarget/ApplicationInsightsTarget.cs

@@ -126,6 +126,11 @@
             base.InitializeTarget();
 
             string connectionString = this.connectionStringLayout.Render(LogEventInfo.CreateNullEvent());
+            if (string.IsNullOrWhiteSpace(connectionString))
+            {
+                connectionString = Environment.GetEnvironmentVariable("APPLICATIONINSIGHTS_CONNECTION_STRING");
+            }
+
             if (string.IsNullOrWhiteSpace(connectionString))
             {
                 throw new NLogConfigurationException(ConnectionStringRequiredMessage);
@@ -187,7 +192,7 @@
         /// <exception cref="ArgumentNullException">If <paramref name="logEvent"/> is null.</exception>
         protected override void Write(LogEventInfo logEvent)
         {
             if (logEvent == null)
             {
                 throw new ArgumentNullException(nameof(logEvent));
             }
@@ -215,7 +220,7 @@
         /// <param name="asyncContinuation">The asynchronous continuation.</param>
         protected override void FlushAsync(AsyncContinuation asyncContinuation)
         {
             if (asyncContinuation == null)
             {
                 throw new ArgumentNullException(nameof(asyncContinuation));
             }