CSHARP-1373: added $max expression support.

Author: craiggwilson

Docs/reference/content/reference/driver/expressions.md

@@ -754,6 +754,15 @@ p => p.FavoriteNumbers.Concat(new [] { 1, 2, 3 })
 { $concatArrays: ['$FavoriteNumbers', [1, 2, 3]] }
 ```
 
+#### $max
+
+```csharp
+p => p.FavoriteNumbers.Max()
+```
+```json
+{ $max: '$FavoriteNumbers' }
+```
+
 #### $min
 
 ```csharp

src/MongoDB.Driver.Tests/Linq/Translators/AggregateProjectTranslatorTests.cs

@@ -463,6 +463,28 @@ public async Task Should_translate_map_with_value()
             result.Value.Result.Should().Equal("it0", "icky0");
         }
 
+        [Test]
+        [RequiresServer(MinimumVersion = "3.1.7")]
+        public async Task Should_translate_max()
+        {
+            var result = await Project(x => new { Result = x.M.Max() });
+
+            result.Projection.Should().Be("{ Result: { \"$max\": \"$M\" }, _id: 0 }");
+
+            result.Value.Result.Should().Be(5);
+        }
+
+        [Test]
+        [RequiresServer(MinimumVersion = "3.1.7")]
+        public async Task Should_translate_max_with_selector()
+        {
+            var result = await Project(x => new { Result = x.G.Max(g => g.E.F) });
+
+            result.Projection.Should().Be("{ Result: { \"$max\": \"$G.E.F\" }, _id: 0 }");
+
+            result.Value.Result.Should().Be(55);
+        }
+
         [Test]
         [RequiresServer(MinimumVersion = "2.4.0")]
         public async Task Should_translate_millisecond()

src/MongoDB.Driver/Linq/Translators/AggregateLanguageTranslator.cs

@@ -396,6 +396,7 @@ private BsonValue TranslatePipeline(PipelineExpression node)
                 TryTranslateAnyResultOperator(node, out result) ||
                 TryTranslateContainsResultOperator(node, out result) ||
                 TryTranslateCountResultOperator(node, out result) ||
+                TryTranslateMaxResultOperator(node, out result) ||
                 TryTranslateMinResultOperator(node, out result))
             {
                 return result;
@@ -624,6 +625,19 @@ private bool TryTranslateHashSetMethodCall(MethodCallExpression node, out BsonVa
             return false;
         }
 
+        private bool TryTranslateMaxResultOperator(PipelineExpression node, out BsonValue result)
+        {
+            var resultOperator = node.ResultOperator as MaxResultOperator;
+            if (resultOperator != null)
+            {
+                result = new BsonDocument("$max", TranslateValue(node.Source));
+                return true;
+            }
+
+            result = null;
+            return false;
+        }
+
         private bool TryTranslateMinResultOperator(PipelineExpression node, out BsonValue result)
         {
             var resultOperator = node.ResultOperator as MinResultOperator;