CSHARP-2824: Collect metadata for legacy API.

Author: DmitryLukyanov

src/MongoDB.Driver.Core/Core/Connections/ClientDocumentHelper.cs

@@ -59,11 +59,22 @@ internal static BsonDocument CreateDriverDocument()
 
         internal static BsonDocument CreateDriverDocument(string driverVersion)
         {
+            var driverName = "mongo-csharp-driver";
+            if (IsLegacyLoaded())
+            {
+                driverName = $"{driverName}|legacy";
+            }
+
             return new BsonDocument
             {
-                { "name", "mongo-csharp-driver" },
+                { "name", driverName },
                 { "version", driverVersion }
             };
+
+            bool IsLegacyLoaded()
+            {
+                return Type.GetType("MongoDB.Driver.MongoServer, MongoDB.Driver.Legacy") != null;
+            }
         }
 
         internal static BsonDocument CreateOSDocument()

tests/MongoDB.Driver.Legacy.Tests/ClientDocumentHelperTests.cs

@@ -0,0 +1,47 @@
+/* Copyright 2020-present MongoDB Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+* http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+using System.Linq;
+using FluentAssertions;
+using MongoDB.Bson;
+using MongoDB.Bson.TestHelpers.XunitExtensions;
+using MongoDB.Driver.Core.Connections;
+using Xunit;
+
+namespace MongoDB.Driver.Legacy.Tests
+{
+    public class ClientDocumentHelperTests
+    {
+        [Fact]
+        public void CreateDriverDocument_should_return_expected_result()
+        {
+            var result = ClientDocumentHelper.CreateDriverDocument();
+
+            result["name"].AsString.Should().Be("mongo-csharp-driver|legacy");
+            result["version"].BsonType.Should().Be(BsonType.String);
+        }
+
+        [Theory]
+        [ParameterAttributeData]
+        public void CreateDriverDocument_with_args_should_return_expected_result(
+            [Values("2.4.0", "2.4.1")]
+            string driverVersion)
+        {
+            var result = ClientDocumentHelper.CreateDriverDocument(driverVersion);
+
+            result.Should().Be($"{{ name : 'mongo-csharp-driver|legacy', version : '{driverVersion}' }}");
+        }
+    }
+}