Add IntakeResponse deserialization for detailed error logging

Introduced the `IntakeResponse` class to better handle and log errors from APM server responses. Updated `PayloadSenderV2` to use this class for extracting error messages and logging them in a more informative way. Added a deserialization method in `PayloadItemSerializer` for converting response streams.

src/Elastic.Apm/Report/IntakeResponse.cs

@@ -0,0 +1,25 @@
+// 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.Collections.Generic;
+using Elastic.Apm.Libraries.Newtonsoft.Json;
+
+namespace Elastic.Apm.Report;
+
+internal class IntakeResponse
+{
+	[JsonProperty("accepted")]
+	public int Accepted { get; set; }
+
+	[JsonProperty("errors")]
+	public IReadOnlyCollection<IntakeError> Errors { get; set; }
+}
+
+internal class IntakeError
+{
+
+	[JsonProperty("message")]
+	public string Message { get; set; }
+}

src/Elastic.Apm/Report/PayloadSenderV2.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using System.Net.Http;
 using System.Net.Http.Headers;
 using System.Text;
@@ -420,14 +421,26 @@ private void ProcessQueueItems(object[] queueItems)
 					// ReSharper disable ConditionIsAlwaysTrueOrFalse
 					if (response is null || !response.IsSuccessStatusCode)
 					{
+						var message = "Unknown 400 Bad Request";
+						if (response?.Content != null)
+						{
+							#if NET6_0_OR_GREATER
+							var intakeResponse = _payloadItemSerializer.Deserialize<IntakeResponse>(response.Content.ReadAsStream());
+#else
+							var intakeResponse = _payloadItemSerializer.Deserialize<IntakeResponse>(response.Content.ReadAsStreamAsync().GetAwaiter().GetResult());
+#endif
+							if (intakeResponse.Errors.Count > 0)
+								message = string.Join(", ", intakeResponse.Errors.Select(e => e.Message).Distinct());
+						}
 						_logger?.Error()
 							?.Log("Failed sending event."
 								+ " Events intake API absolute URL: {EventsIntakeAbsoluteUrl}."
 								+ " APM Server response: status code: {ApmServerResponseStatusCode}"
-								+ ", content: \n{ApmServerResponseContent}"
+								+ ", reasons: {ApmServerResponseContent}"
 								, _intakeV2EventsAbsoluteUrl.Sanitize()
 								, response?.StatusCode,
-								response?.Content.ReadAsStringAsync().ConfigureAwait(false).GetAwaiter().GetResult());
+								message
+								);
 					}
 					// ReSharper enable ConditionIsAlwaysTrueOrFalse
 					else

src/Elastic.Apm/Report/Serialization/PayloadItemSerializer.cs

@@ -42,6 +42,14 @@ internal T Deserialize<T>(string json)
 			return val ?? default;
 		}
 
+		internal T Deserialize<T>(Stream stream)
+		{
+			using var sr = new StreamReader(stream);
+			using var jsonTextReader = new JsonTextReader(sr);
+			var val = _serializer.Deserialize<T>(jsonTextReader);
+			return val;
+		}
+
 		/// <summary>
 		/// Serializes the item to JSON
 		/// </summary>