No longer parse request cookies, but ensure they are still redacted in the Cookie header string

docs/configuration.asciidoc

@@ -469,8 +469,7 @@ See https://www.owasp.org/index.php/Information_exposure_through_query_strings_i
 *`Cookie` header sanitization:*
 
 The `Cookie` header is automatically redacted for incoming HTTP request transactions. Each name-value pair from the 
-Cookie header is parsed by the agent and sent to the APM Server. Before the name-value pairs are recorded, they are 
-sanitized based on the `SanitizeFieldNames` configuration. Cookies with sensitive data in 
+cookie list is parsed by the agent and sanitized based on the `SanitizeFieldNames` configuration. Cookies with sensitive data in 
 their value can be redacted by adding the cookie's name to the comma-separated list.
 
 [options="header"]

src/Elastic.Apm/Api/Request.cs

@@ -27,11 +27,6 @@ public class Request
 		/// </summary>
 		public Dictionary<string, string> Headers { get; set; }
 
-		/// <summary>
-		/// This field is sanitized by a filter
-		/// </summary>
-		public Dictionary<string, string> Cookies { get; set; }
-
 		[JsonProperty("http_version")]
 		[MaxLength]
 		public string HttpVersion { get; set; }
@@ -47,8 +42,6 @@ internal Request DeepCopy()
 			var newItem = (Request)MemberwiseClone();
 			if (Headers != null)
 				newItem.Headers = Headers.ToDictionary(entry => entry.Key, entry => entry.Value);
-			if (Cookies != null)
-				newItem.Cookies = Cookies.ToDictionary(entry => entry.Key, entry => entry.Value);
 
 			newItem.Socket = Socket?.DeepCopy();
 			newItem.Url = Url?.DeepCopy();

src/Elastic.Apm/Filters/CookieHeaderRedactionFilter.cs

@@ -0,0 +1,91 @@
+// Licensed to Elasticsearch B.V under one or more agreements.
+// Elasticsearch B.V licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using Elastic.Apm.Api;
+using Elastic.Apm.Config;
+using Elastic.Apm.Helpers;
+using Elastic.Apm.Model;
+#if NET6_0_OR_GREATER
+using System.Buffers;
+#endif
+
+namespace Elastic.Apm.Filters
+{
+	/// <summary>
+	/// Redacts items from the cookie list of the Cookie request header.
+	/// </summary>
+	internal class CookieHeaderRedactionFilter
+	{
+		private const string CookieHeader = "Cookie";
+
+		public static IError Filter(IError error)
+		{
+			if (error is Error e && e.Context is not null)
+				HandleCookieHeader(e.Context?.Request?.Headers, e.Configuration.SanitizeFieldNames);
+			return error;
+		}
+
+		public static ITransaction Filter(ITransaction transaction)
+		{
+			if (transaction is Transaction { IsContextCreated: true })
+				HandleCookieHeader(transaction.Context?.Request?.Headers, transaction.Configuration.SanitizeFieldNames);
+			return transaction;
+		}
+
+		// internal for testing
+		internal static void HandleCookieHeader(Dictionary<string, string> headers, IReadOnlyList<WildcardMatcher> sanitizeFieldNames)
+		{
+			if (headers is not null)
+			{
+				// Realistically, this should be more than enough for all sensible scenarios
+				// e.g. Cookies | cookies | COOKIES
+				const int maxKeys = 4;
+
+#if NET6_0_OR_GREATER
+				var matchedKeys = ArrayPool<string>.Shared.Rent(maxKeys);
+				var matchedValues = ArrayPool<string>.Shared.Rent(maxKeys);
+#else
+				var matchedKeys = new string[maxKeys];
+				var matchedValues = new string[maxKeys];
+#endif
+				var matchedCount = 0;
+
+				foreach (var header in headers)
+				{
+					if (matchedCount == maxKeys)
+						break;
+
+					if (header.Key.Equals(CookieHeader, StringComparison.OrdinalIgnoreCase))
+					{
+						matchedKeys[matchedCount] = header.Key;
+						matchedValues[matchedCount] = CookieHeaderRedacter.Redact(header.Value, sanitizeFieldNames);
+						matchedCount++;
+					}
+				}
+
+				var replacedCount = 0;
+
+				foreach (var headerKey in matchedKeys)
+				{
+					if (replacedCount == matchedCount)
+						break;
+
+					if (headerKey is not null)
+					{
+						headers[headerKey] = matchedValues[replacedCount];
+						replacedCount++;
+					}
+				}
+
+#if NET6_0_OR_GREATER
+				ArrayPool<string>.Shared.Return(matchedKeys);
+				ArrayPool<string>.Shared.Return(matchedValues);
+#endif
+			}
+		}
+	}
+}

src/Elastic.Apm/Filters/Sanitization.cs

@@ -18,9 +18,6 @@ public static void SanitizeHeadersInContext(Context context, IConfiguration conf
 			if (context?.Request?.Headers is not null)
 				RedactMatches(context?.Request?.Headers, configuration);
 
-			if (context?.Request?.Cookies is not null)
-				RedactMatches(context?.Request?.Cookies, configuration);
-
 			if (context?.Response?.Headers is not null)
 				RedactMatches(context?.Response?.Headers, configuration);