Add monitor to keep track of current value of App Services env vars

Src/WindowsServer/WindowsServer.Shared/AzureWebAppRoleEnvironmentTelemetryInitializer.cs

@@ -44,8 +44,7 @@ public void Initialize(ITelemetry telemetry)
 
             if (string.IsNullOrEmpty(telemetry.Context.GetInternalContext().NodeName))
             {
-                string name = LazyInitializer.EnsureInitialized(ref this.nodeName, this.GetNodeName);                
-                telemetry.Context.GetInternalContext().NodeName = name;
+                telemetry.Context.GetInternalContext().NodeName = this.GetNodeName();
             }
         }
 
@@ -62,7 +61,8 @@ private string GetRoleName()
 
         private string GetNodeName()
         {
-            return Environment.GetEnvironmentVariable(WebAppHostNameEnvironmentVariable) ?? string.Empty;
+            AppServiceEnvVarMonitor.GetUpdatedEnvironmentVariable(WebAppHostNameEnvironmentVariable, ref this.nodeName);
+            return this.nodeName;
         }
     }
 }

Src/WindowsServer/WindowsServer.Shared/Implementation/AppServiceEnvVarMonitor.cs

@@ -0,0 +1,57 @@
+namespace Microsoft.ApplicationInsights.WindowsServer.Implementation
+{
+    using System;
+    using System.Collections.Generic;
+
+    /// <summary>
+    /// Utility to monitor the value of environment variables which may change 
+    /// during the run of an application. Checks the environment variables 
+    /// intermittently.
+    /// </summary>
+    internal static class AppServiceEnvVarMonitor
+    {
+        internal static TimeSpan CheckInterval = TimeSpan.FromSeconds(30);
+        internal static DateTime LastCheckTime = DateTime.MinValue;
+
+        /// <summary>
+        /// Environment variables tracked by this monitor.
+        /// </summary>
+        internal static Dictionary<string, string> CheckedValues = new Dictionary<string, string>()
+        {
+            { "WEBSITE_SITE_NAME", string.Empty },
+            { "WEBSITE_HOME_STAMPNAME", string.Empty },
+            {  "WEBSITE_HOSTNAME", string.Empty }
+        };
+
+        /// <summary>
+        /// Get the latest value assigned to an environment variable.
+        /// </summary>
+        /// <param name="envVarName">Name of the environment variable to acquire.</param>
+        /// <param name="value">Value of the environment variable (updated within the update interval).</param>
+        public static void GetUpdatedEnvironmentVariable(string envVarName, ref string value)
+        {
+            if (!string.IsNullOrEmpty(envVarName))
+            {
+                CheckVariablesIntermittent();
+                CheckedValues.TryGetValue(envVarName, out value);
+            }
+        }
+
+        /// <summary>
+        /// Simply update the stored environment variables if the last time we 
+        /// checked from now is greater than the check interval.
+        /// </summary>
+        private static void CheckVariablesIntermittent()
+        {
+            DateTime checkTime = DateTime.Now;
+            if (checkTime - LastCheckTime > CheckInterval)
+            {
+                LastCheckTime = checkTime;
+                foreach (var kvp in CheckedValues)
+                {
+                    CheckedValues[kvp.Key] = Environment.GetEnvironmentVariable(kvp.Key);
+                }
+            }
+        }
+    }
+}

Src/WindowsServer/WindowsServer.Shared/WindowsServer.Shared.projitems

@@ -15,6 +15,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)BuildInfoConfigComponentVersionTelemetryInitializer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)DeveloperModeWithDebuggerAttachedTelemetryModule.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)DomainNameRoleInstanceTelemetryInitializer.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Implementation\AppServiceEnvVarMonitor.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Implementation\AzureComputeMetadataHeartbeatPropertyProvider.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Implementation\AzureMetadataRequestor.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Implementation\DataContracts\AzureInstanceComputeMetadata.cs" />