fix: improve handling of .NET environments and settings from appsettings.*.json files (#2125)

* POC fix

* Take DOTNET_ENVIRONMENT into account

Also refactor

* Refactor based on team pairing

* Fix integration test

Author: nr-ahemsath

src/Agent/NewRelic/Agent/Core/Configuration/AppSettingsConfigResolveWhenUsed.cs

@@ -3,6 +3,7 @@
 
 #if NETSTANDARD2_0
 using System;
+using System.Collections.Generic;
 using System.IO;
 using Microsoft.Extensions.Configuration;
 using NewRelic.Core;
@@ -39,30 +40,42 @@ private static IConfigurationRoot InitializeConfiguration()
                 applicationDirectory = Directory.GetCurrentDirectory();
             }
 
+            // add default appsettings.json files to config builder
             var builder = new ConfigurationBuilder()
                 .SetBasePath(applicationDirectory)
                 .AddJsonFile("appsettings.json", optional: true, reloadOnChange: false);
 
-            // Determine if there might be an environment-specific appsettings file
-            var env = new SystemInterfaces.Environment();
-            var environment = env.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT");
-            if (string.IsNullOrEmpty(environment))
-            {
-                environment = env.GetEnvironmentVariable("EnvironmentName");
-            }
+            _appSettingsFilePaths = Path.Combine(applicationDirectory, "appsettings.json");
 
-            if (!string.IsNullOrEmpty(environment))
-            {
-                builder.AddJsonFile($"appsettings.{environment}.json", optional: true, reloadOnChange: false);
-            }
-
-            var appSettingsPath = Path.Combine(applicationDirectory, "appsettings.json");
+            // Determine if there is a .NET environment configured, or default to "Production"
+            var environment = GetDotnetEnvironment();
+            builder.AddJsonFile($"appsettings.{environment}.json", optional: true, reloadOnChange: false);
             var appSettingsEnvPath = Path.Combine(applicationDirectory, $"appsettings.{environment}.json");
-            _appSettingsFilePaths = !string.IsNullOrEmpty(environment) ? string.Join(", ", appSettingsPath, appSettingsEnvPath) : appSettingsPath;
+            _appSettingsFilePaths = string.Join(", ", _appSettingsFilePaths, appSettingsEnvPath);
 
             return builder.Build();
         }
 
+        private static string GetDotnetEnvironment()
+        {
+            var env = new SystemInterfaces.Environment();
+            // Determine the environment (e.g. Production, Development, Staging, etc.) by considering the following env vars in order
+            // "DOTNET_ENVIRONMENT" takes precedence over "ASPNETCORE_ENVIRONMENT", even for ASP.NET Core applications
+            // EnvironmentName is proprietary to our agent and the behavior as of version 10.20 is to not take precedence over the .NET builtins
+            var envVarsToCheck = new List<string>() { "DOTNET_ENVIRONMENT", "ASPNETCORE_ENVIRONMENT", "EnvironmentName" };
+            foreach ( var envVar in envVarsToCheck )
+            {
+                var environment = env.GetEnvironmentVariable(envVar);
+                if (!string.IsNullOrEmpty(environment))
+                {
+                    Log.Debug($".NET environment set to '{environment}' from env var '{envVar}'");
+                    return environment;
+                }
+            }
+            Log.Finest("No .NET environment configured in DOTNET_ENVIRONMENT, ASPNETCORE_ENVIRONMENT, or EnvironmentName. Defaulting to 'Production'");
+            return "Production";
+        }
+
         public static string GetAppSetting(string key)
         {
             if (key == null)

tests/Agent/IntegrationTests/IntegrationTests/AgentFeatures/EnvironmentTests.cs

@@ -60,7 +60,7 @@ public void TestConfigPaths()
             _connectData = _connectData ?? _fixture.AgentLog.GetConnectData();
 
             var nrConfig = _connectData?.Environment?.GetPropertyString("Initial NewRelic Config");
-            var appConfig = _connectData?.Environment?.GetPropertyString("Application Config");
+            var appConfig = _connectData?.Environment?.GetPropertyString("Application Config").Split(',')[0];
 
             NrAssert.Multiple(
                 () => Assert.NotNull(nrConfig),