Backport transcoding fixes to 5.0

vertx-grpc-common/src/main/java/io/vertx/grpc/common/GrpcMessageDecoder.java

@@ -10,11 +10,13 @@
  */
 package io.vertx.grpc.common;
 
+import com.google.protobuf.Descriptors;
 import com.google.protobuf.InvalidProtocolBufferException;
 import com.google.protobuf.Message;
 import com.google.protobuf.MessageOrBuilder;
 import com.google.protobuf.Parser;
 import com.google.protobuf.util.JsonFormat;
+import io.vertx.codegen.annotations.Unstable;
 import io.vertx.core.buffer.Buffer;
 import io.vertx.core.json.DecodeException;
 import io.vertx.core.json.Json;
@@ -60,6 +62,10 @@ public T decode(GrpcMessage msg) throws CodecException {
       public boolean accepts(WireFormat format) {
         return true;
       }
+      @Override
+      public Descriptors.Descriptor messageDescriptor() {
+        return dit.getDescriptorForType();
+      }
     };
   }
 
@@ -165,8 +171,31 @@ public boolean accepts(WireFormat format) {
     }
   };
 
+  /**
+   * Decodes a given gRPC message into an object of type {@code T}.
+   *
+   * @param msg the gRPC message to decode
+   * @return the decoded message of type {@code T}
+   * @throws CodecException if the decoding process fails
+   */
   T decode(GrpcMessage msg) throws CodecException;
 
+  /**
+   * Determines whether the given {@link WireFormat} is supported by this decoder.
+   *
+   * @param format the wire format to check
+   * @return true if the given wire format is accepted, false otherwise
+   */
   boolean accepts(WireFormat format);
 
+  /**
+   * Returns the protobuf message descriptor if this decoder was created from a protobuf message type, {@code null} otherwise.
+   *
+   * @return the protobuf descriptor or {@code null}
+   */
+  @Unstable
+  default Descriptors.Descriptor messageDescriptor() {
+    return null;
+  }
+
 }