fix: Retry connection on HttpRequestException error (#1514) (#1484)

Author: tippmar-nr

src/Agent/NewRelic/Agent/Core/DataTransport/ConnectionManager.cs

@@ -10,6 +10,7 @@
 using System.IO;
 using System.Linq;
 using System.Net;
+using System.Net.Http;
 using System.Net.Sockets;
 
 namespace NewRelic.Agent.Core.DataTransport
@@ -106,6 +107,12 @@ private void Connect()
             {
                 HandleHttpErrorResponse(ex);
             }
+            // Occurs when the agent is unable to connect to APM. The request failed due to an underlying
+            // issue such as network connectivity, DNS failure, server certificate validation or timeout.
+            catch (HttpRequestException)
+            {
+                ScheduleRestart();
+            }
             // Occurs when the agent connects to APM but the connection gets aborted by the collector
             catch (SocketException)
             {

tests/Agent/UnitTests/Core.UnitTest/DataTransport/ConnectionManagerTests.cs

@@ -4,15 +4,17 @@
 using System;
 using System.IO;
 using System.Net;
+using System.Net.Http;
 using System.Net.Sockets;
+using System.Web;
 using NewRelic.Agent.Configuration;
 using NewRelic.Agent.Core.Events;
-using NewRelic.Agent.Core.Exceptions;
 using NewRelic.Agent.Core.Fixtures;
 using NewRelic.Agent.Core.Time;
 using NewRelic.Agent.Core.Utilities;
 using NUnit.Framework;
 using Telerik.JustMock;
+using HttpException = NewRelic.Agent.Core.Exceptions.HttpException;
 
 namespace NewRelic.Agent.Core.DataTransport
 {
@@ -83,6 +85,7 @@ public void AttemptAutoStart_SchedulesConnectAsynchronously_IfAutoStartIsOnAndSy
         [Test]
         [TestCase("ForceRestartException")]
         [TestCase("HttpException")]
+        [TestCase("HttpRequestException")]
         [TestCase("SocketException")]
         [TestCase("IOException")]
         [TestCase("OperationCanceledException")]
@@ -97,6 +100,9 @@ public void AttemptAutoStart_SchedulesReconnect_IfCertainExceptionOccurs(string
                 case "HttpException":
                     ex = new HttpException(HttpStatusCode.MethodNotAllowed, null);
                     break;
+                case "HttpRequestException":
+                    ex = new HttpRequestException();
+                    break;
                 case "SocketException":
                     ex = new SocketException();
                     break;