Aggregations: More consistent response format for scripted metrics aggregation

Changes the name of the field in the scripted metrics aggregation from 'aggregation' to 'value' to be more in line with the other metrics aggregations like 'avg'

docs/reference/search/aggregations/metrics/scripted-metric-aggregation.asciidoc

@@ -46,7 +46,7 @@ The response for the above aggregation:
 
     "aggregations": {
         "profit": {
-            "aggregation": 170
+            "value": 170
         }
    }
 }
@@ -207,7 +207,7 @@ produce the response:
 
     "aggregations": {
         "profit": {
-            "aggregation": 170
+            "value": 170
         }
    }
 }

src/main/java/org/elasticsearch/search/aggregations/metrics/scripted/InternalScriptedMetric.java

@@ -134,7 +134,7 @@ public class InternalScriptedMetric extends InternalMetricsAggregation implement
 
     @Override
     public XContentBuilder doXContentBody(XContentBuilder builder, Params params) throws IOException {
-        return builder.field("aggregation", aggregation);
+        return builder.field("value", aggregation);
     }
 
 }