Fix `metric` impl on unmapped fields.

src/main/java/org/elasticsearch/search/aggregations/metrics/avg/AvgAggregator.java

@@ -80,7 +80,7 @@ public class AvgAggregator extends MetricsAggregator.SingleValue {
 
     @Override
     public double metric(long owningBucketOrd) {
-        return sums.get(owningBucketOrd) / counts.get(owningBucketOrd);
+        return valuesSource == null ? Double.NaN : sums.get(owningBucketOrd) / counts.get(owningBucketOrd);
     }
 
     @Override

src/main/java/org/elasticsearch/search/aggregations/metrics/max/MaxAggregator.java

@@ -80,7 +80,7 @@ public class MaxAggregator extends MetricsAggregator.SingleValue {
 
     @Override
     public double metric(long owningBucketOrd) {
-        return maxes.get(owningBucketOrd);
+        return valuesSource == null ? Double.NEGATIVE_INFINITY : maxes.get(owningBucketOrd);
     }
 
     @Override

src/main/java/org/elasticsearch/search/aggregations/metrics/min/MinAggregator.java

@@ -79,7 +79,7 @@ public class MinAggregator extends MetricsAggregator.SingleValue {
 
     @Override
     public double metric(long owningBucketOrd) {
-        return mins.get(owningBucketOrd);
+        return valuesSource == null ? Double.POSITIVE_INFINITY : mins.get(owningBucketOrd);
     }
 
     @Override

src/main/java/org/elasticsearch/search/aggregations/metrics/sum/SumAggregator.java

@@ -75,7 +75,7 @@ public class SumAggregator extends MetricsAggregator.SingleValue {
 
     @Override
     public double metric(long owningBucketOrd) {
-        return sums.get(owningBucketOrd);
+        return valuesSource == null ? 0 : sums.get(owningBucketOrd);
     }
 
     @Override

src/main/java/org/elasticsearch/search/aggregations/metrics/valuecount/ValueCountAggregator.java

@@ -74,7 +74,7 @@ public class ValueCountAggregator extends MetricsAggregator.SingleValue {
 
     @Override
     public double metric(long owningBucketOrd) {
-        return counts.get(owningBucketOrd);
+        return valuesSource == null ? 0 : counts.get(owningBucketOrd);
     }
 
     @Override