[Backport 6x] Fix check for agressive connectionlimit (#4669)

In dotnet/runtime#22366 we found that if
`HttpClient` is using the curl handler it will lead to TCP connections
bleeding. Our DefaultConnectionLimit is very restrictive if this is
true. Our check however is too lenient and will default to true always
on .NET core since netcoreapp still ships with CurlHandler as recent as
`3.1.x`

Author: Mpdreamz

src/Elasticsearch.Net/Configuration/ConnectionConfiguration.cs

@@ -22,6 +22,43 @@ namespace Elasticsearch.Net
 	/// </summary>
 	public class ConnectionConfiguration : ConnectionConfiguration<ConnectionConfiguration>
 	{
+		/// <summary>
+		/// Detects whether we are running on .NET Core with CurlHandler.
+		/// If this is true, we will set a very restrictive <see cref="DefaultConnectionLimit"/>
+		/// As the old curl based handler is known to bleed TCP connections:
+		/// <para>https://github.com/dotnet/runtime/issues/22366</para>
+		/// </summary>
+        private static bool UsingCurlHandler
+		{
+			get
+			{
+#if !DOTNETCORE
+				return false;
+#else
+				var curlHandlerExists = typeof(HttpClientHandler).Assembly.GetType("System.Net.Http.CurlHandler") != null;
+				if (!curlHandlerExists) return false;
+
+				var socketsHandlerExists = typeof(HttpClientHandler).Assembly.GetType("System.Net.Http.SocketsHttpHandler") != null;
+				// running on a .NET core version with CurlHandler, before the existence of SocketsHttpHandler.
+				// Must be using CurlHandler.
+				if (!socketsHandlerExists) return true;
+
+				if (AppContext.TryGetSwitch("System.Net.Http.UseSocketsHttpHandler", out var isEnabled))
+					return !isEnabled;
+
+				var environmentVariable =
+					Environment.GetEnvironmentVariable("DOTNET_SYSTEM_NET_HTTP_USESOCKETSHTTPHANDLER");
+
+				// SocketsHandler exists and no environment variable exists to disable it.
+				// Must be using SocketsHandler and not CurlHandler
+				if (environmentVariable == null) return false;
+
+				return environmentVariable.Equals("false", StringComparison.OrdinalIgnoreCase) ||
+					environmentVariable.Equals("0");
+#endif
+			}
+		}
+
 		/// <summary>
 		/// The default ping timeout. Defaults to 2 seconds
 		/// </summary>
@@ -40,12 +77,12 @@ public class ConnectionConfiguration : ConnectionConfiguration<ConnectionConfigu
 		public static readonly TimeSpan DefaultTimeout = TimeSpan.FromMinutes(1);
 
 		/// <summary>
-		/// The default connection limit for both Elasticsearch.Net and Nest. Defaults to <c>80</c> except for
-		/// HttpClientHandler implementations based on curl, which defaults to
-		/// <see cref="Environment.ProcessorCount" />
+		/// The default connection limit for both Elasticsearch.Net and Nest. Defaults to <c>80</c>
+#if DOTNETCORE
+		/// <para>Except for <see cref="HttpClientHandler"/> implementations based on curl, which defaults to <see cref="Environment.ProcessorCount"/></para>
+#endif
 		/// </summary>
-		public static readonly int DefaultConnectionLimit = IsCurlHandler ? Environment.ProcessorCount : 80;
-
+		public static readonly int DefaultConnectionLimit = UsingCurlHandler ? Environment.ProcessorCount : 80;
 
 		/// <summary>
 		/// The default user agent for Elasticsearch.Net