feat!: Expose error details as synthesized gRPC trailers in REGAPIC

Unit tests will be in a later commit. Currently tested via making a deliberately-failing call to the Translation API.

BREAKING CHANGE: previous error messages containing large amounts of
JSON will be replaced with simpler ones, with the detail available
via the RpcException extension methods. This could break anyone
using REGAPIC (currently just Compute) and trying to find the error
details in the message. (This change will only be in a new major
version anyway, but we should include this in the change log.)

Google.Api.Gax.Grpc/Rest/ReadHttpResponseMessage.cs

@@ -5,7 +5,10 @@
  * https://developers.google.com/open-source/licenses/bsd
  */
 
+using Google.Protobuf;
+using Google.Protobuf.Reflection;
 using Grpc.Core;
+using System.Net;
 using System.Net.Http;
 using System.Runtime.ExceptionServices;
 
@@ -27,8 +30,11 @@ namespace Google.Api.Gax.Grpc.Rest
     /// </summary>
     internal class ReadHttpResponseMessage
     {
+        private static readonly JsonParser s_responseMetadataParser = new JsonParser(
+            JsonParser.Settings.Default.WithIgnoreUnknownFields(true).WithTypeRegistry(TypeRegistry.FromFiles(Rpc.ErrorDetailsReflection.Descriptor)));
         private HttpResponseMessage OriginalResponseMessage { get; }
 
+        private readonly Rpc.Status _rpcStatus;
         private readonly string _content;
         private readonly ExceptionDispatchInfo _readException;
 
@@ -42,9 +48,11 @@ internal string Content
         }
 
         internal ReadHttpResponseMessage(HttpResponseMessage response, string content) =>
-            (OriginalResponseMessage, _content) = (response, content);
+            (OriginalResponseMessage, _content, _rpcStatus) = (response, content, CreateRpcStatus(response.StatusCode, content));
 
         internal ReadHttpResponseMessage(HttpResponseMessage response, ExceptionDispatchInfo readException) =>
+            // If we didn't manage to read the response, we don't have any information to create a fake status code from.
+            // We'll bubble up the _readException instead.
             (OriginalResponseMessage, _readException) = (response, readException);
 
         internal Metadata GetHeaders()
@@ -60,15 +68,55 @@ internal Metadata GetHeaders()
 
         internal Status GetStatus()
         {
-            var grpcStatus = RestGrpcAdapter.ConvertHttpStatusCode((int) OriginalResponseMessage.StatusCode);
-            return new Status(grpcStatus,
-                // Notice that here, if there was an exception reading the content
-                // we'll bubble it up. This is similar to what's done if there's an
-                // exception while sending the request, and if there's an exception
-                // reading the content for TResponse.
-                grpcStatus == StatusCode.OK ? "" : Content);
+            // Notice that here, if there was an exception reading the content
+            // we'll bubble it up. This is similar to what's done if there's an
+            // exception while sending the request, and if there's an exception
+            // reading the content for TResponse.
+            _readException?.Throw();
+            return new Status((StatusCode) _rpcStatus.Code, _rpcStatus.Message);
         }
 
-        internal Metadata GetTrailers() => new Metadata(); // We never have any trailers.
+        internal Metadata GetTrailers()
+        {
+            if (_rpcStatus is null || _rpcStatus.Code == (int) Rpc.Code.Ok)
+            {
+                return new Metadata();
+            }
+            return new Metadata { { RpcExceptionExtensions.StatusDetailsTrailerName, _rpcStatus.ToByteArray() } };
+        }
+
+        /// <summary>
+        /// Create an RPC status from the HTTP status code, attempting to parse the
+        /// content as an Rpc.Status if the HTTP status indicates a failure.
+        /// </summary>
+        internal static Rpc.Status CreateRpcStatus(HttpStatusCode statusCode, string content)
+        {
+            var grpcStatusCode = RestGrpcAdapter.ConvertHttpStatusCode((int) statusCode);
+            if (grpcStatusCode == StatusCode.OK)
+            {
+                return new Rpc.Status { Code = (int) grpcStatusCode };
+            }
+            try
+            {
+                var errorFromMetadata = s_responseMetadataParser.Parse<Error>(content);
+                var error = errorFromMetadata.Error_;
+                if (error is null)
+                {
+                    return new Rpc.Status { Code = (int) grpcStatusCode };
+                }
+                var status = new Rpc.Status
+                {
+                    Code = (int) error.Status_,
+                    Message = error.Message,
+                    Details = { error.Details }
+                };
+                return status;
+            }
+            catch
+            {
+                // If we can't parse the result as JSON, just use the content as the error message.
+                return new Rpc.Status { Code = (int) grpcStatusCode, Message = content };
+            }
+        }
     }
 }

Google.Api.Gax.Grpc/RpcExceptionExtensions.cs

@@ -19,7 +19,7 @@ namespace Google.Api.Gax.Grpc
     /// </summary>
     public static class RpcExceptionExtensions
     {
-        // Visible for testing.
+        // Visible for testing, and for synthesis in ReadHttpResponseMessage.
         internal const string StatusDetailsTrailerName = "grpc-status-details-bin";
 
         /// <summary>