CSHARP-3915: Implemented Densify Builder and LINQ methods.

Author: rstam

src/MongoDB.Driver/AggregateFluent.cs

@@ -106,6 +106,22 @@ public override IAggregateFluent<AggregateCountResult> Count()
             return WithPipeline(_pipeline.Count());
         }
 
+        public override IAggregateFluent<TResult> Densify(
+            FieldDefinition<TResult> field,
+            DensifyRange range,
+            IEnumerable<FieldDefinition<TResult>> partitionByFields = null)
+        {
+            return WithPipeline(_pipeline.Densify(field, range, partitionByFields));
+        }
+
+        public override IAggregateFluent<TResult> Densify(
+            FieldDefinition<TResult> field,
+            DensifyRange range,
+            params FieldDefinition<TResult>[] partitionByFields)
+        {
+            return WithPipeline(_pipeline.Densify(field, range, partitionByFields));
+        }
+
         public override IAggregateFluent<TNewResult> Facet<TNewResult>(
             IEnumerable<AggregateFacet<TResult>> facets,
             AggregateFacetOptions<TNewResult> options = null)

src/MongoDB.Driver/AggregateFluentBase.cs

@@ -97,6 +97,24 @@ public virtual IAggregateFluent<AggregateCountResult> Count()
             throw new NotImplementedException();
         }
 
+        /// <inheritdoc />
+        public virtual IAggregateFluent<TResult> Densify(
+            FieldDefinition<TResult> field,
+            DensifyRange range,
+            IEnumerable<FieldDefinition<TResult>> partitionByFields = null)
+        {
+            throw new NotImplementedException();
+        }
+
+        /// <inheritdoc />
+        public virtual IAggregateFluent<TResult> Densify(
+            FieldDefinition<TResult> field,
+            DensifyRange range,
+            params FieldDefinition<TResult>[] partitionByFields)
+        {
+            throw new NotImplementedException();
+        }
+
         /// <inheritdoc />
         public virtual IAggregateFluent<TNewResult> Facet<TNewResult>(
             IEnumerable<AggregateFacet<TResult>> facets,