protobuf-net#1149 Support deprecated options

- Updated `MetaType` to check if `System.ObsoleteAttribute` exists on type or field to add deprecated option while generating proto.
- Added unit test to reproduce the issue.

src/protobuf-net.Test/Issues/Issue1149.cs

@@ -0,0 +1,67 @@
+using System;
+using ProtoBuf.Meta;
+using Xunit;
+
+namespace ProtoBuf.Test.Issues
+{
+#pragma warning disable CS0612 // Type or member is obsolete
+    public sealed class Issue1149
+    {
+        [Fact]
+        public void GenerateProtoWithDeprecatedOption()
+        {
+            const string expected = @"syntax = ""proto3"";
+package DefaultPackage;
+
+message SomeClass {
+   option deprecated = true;
+   int32 SomeProperty = 1;
+}
+enum SomeEnum {
+   option deprecated = true;
+   One = 0;
+   Two = 1;
+   Three = 2;
+}
+message SomeOtherClass {
+   string SomeOtherProperty = 1 [deprecated = true];
+}
+";
+
+            var actual = Serializer.GetProto(new SchemaGenerationOptions
+            {
+                Syntax = ProtoSyntax.Proto3,
+                Package = "DefaultPackage",
+                Types = { typeof(SomeClass), typeof(SomeOtherClass), typeof(SomeEnum) },
+            });
+
+            Assert.Equal(expected, actual);
+        }
+
+        [ProtoContract]
+        [Obsolete]
+        public class SomeClass
+        {
+            [ProtoMember(tag: 1)]
+            public int SomeProperty { get; set; }
+        }
+
+        [ProtoContract]
+        public class SomeOtherClass
+        {
+            [ProtoMember(tag: 1)]
+            [Obsolete]
+            public string SomeOtherProperty { get; set; }
+        }
+
+        [ProtoContract]
+        [Obsolete]
+        public enum SomeEnum
+        {
+            One,
+            Two,
+            Three,
+        }
+    }
+#pragma warning restore CS0612 // Type or member is obsolete
+}

src/protobuf-net/Meta/MetaType.cs

@@ -2041,6 +2041,7 @@ public bool IsGroup
                 if (!allValid) NewLine(builder, indent).Append("/* for context only");
                 NewLine(builder, indent).Append("enum ").Append(GetSchemaTypeName(callstack)).Append(" {");
                 AddNamespace(imports);
+                AppendMessageDeprecatedOption(builder, indent);
 
                 if (Type.IsDefined(typeof(FlagsAttribute), true))
                 {
@@ -2109,6 +2110,7 @@ public bool IsGroup
 
                 NewLine(builder, indent).Append("message ").Append(GetSchemaTypeName(callstack)).Append(" {");
                 AddNamespace(imports);
+                AppendMessageDeprecatedOption(builder, indent);
                 foreach (ValueMember member in fieldsArr)
                 {
                     string schemaTypeName;
@@ -2216,6 +2218,10 @@ void WriteValueMember(string schemaModelTypeName, bool hasGroupModifier = false)
                                 imports.Add(RuntimeTypeModel.CommonImports.Protogen);
                                 AddOption(builder, ref hasOption).Append("(.protobuf_net.fieldopt).dynamicType = true");
                             }
+                            if (member.Member.GetCustomAttribute<ObsoleteAttribute>() != null)
+                            {
+                                AddOption(builder, ref hasOption).Append("deprecated = true");
+                            }
                             CloseOption(builder, ref hasOption).Append(';');
                             if (syntax != ProtoSyntax.Proto2 && member.DefaultValue is not null && !member.IsRequired)
                             {
@@ -2345,6 +2351,13 @@ void AppendReservations()
                         builder.Append(" /* ").Append(reservation.Comment).Append(" */");
                 }
             }
+            void AppendMessageDeprecatedOption(StringBuilder builder, int indent)
+            {
+                if (Type.GetCustomAttribute<ObsoleteAttribute>() != null)
+                {
+                    NewLine(builder, indent + 1).Append("option deprecated = true;");
+                }
+            }
         }
 
         private static StringBuilder AddOption(StringBuilder builder, ref bool hasOption)