Handle "NaN" in sampling fields (#5404)

Fixes #5007

Author: russcam

src/Nest/Aggregations/AggregateFormatter.cs

@@ -639,6 +639,8 @@ private IAggregate GetExtendedStatsAggregate(ref JsonReader reader, IJsonFormatt
 			extendedStatsMetric.SumOfSquares = reader.ReadNullableDouble();
 			reader.ReadNext(); // ,
 
+			var formatter = new NullableStringDoubleFormatter();
+
 			while (reader.GetCurrentJsonToken() != JsonToken.EndObject)
 			{
 				var propertyName = reader.ReadPropertyNameSegmentRaw();
@@ -660,13 +662,13 @@ private IAggregate GetExtendedStatsAggregate(ref JsonReader reader, IJsonFormatt
 							extendedStatsMetric.VariancePopulation = reader.ReadNullableDouble();
 							break;
 						case 4:
-							extendedStatsMetric.VarianceSampling = reader.ReadNullableDouble();
+							extendedStatsMetric.VarianceSampling = formatter.Deserialize(ref reader, formatterResolver);
 							break;
 						case 5:
 							extendedStatsMetric.StdDeviationPopulation = reader.ReadNullableDouble();
 							break;
 						case 6:
-							extendedStatsMetric.StdDeviationSampling = reader.ReadNullableDouble();
+							extendedStatsMetric.StdDeviationSampling = formatter.Deserialize(ref reader, formatterResolver);
 							break;
 					}
 				}

src/Nest/Aggregations/Metric/ExtendedStats/ExtendedStatsAggregate.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information
 
 using System.Runtime.Serialization;
+using Nest.Utf8Json;
 
 namespace Nest
 {
@@ -73,9 +74,11 @@ public class StandardDeviationBounds
 		public double? UpperPopulation { get; set; }
 
 		[DataMember(Name = "lower_sampling")]
+		[JsonFormatter(typeof(NullableStringDoubleFormatter))]
 		public double? LowerSampling { get; set; }
 
 		[DataMember(Name = "upper_sampling")]
+		[JsonFormatter(typeof(NullableStringDoubleFormatter))]
 		public double? UpperSampling { get; set; }
 	}
 }