Change token for nodes stats adaptive selection submetric (#54193)

A poorly-named token was chosen for internal serialization of the
NodesStatsRequest class, and is changed here to a more desirable
value.

server/src/main/java/org/elasticsearch/action/admin/cluster/node/stats/NodesStatsRequest.java

@@ -331,7 +331,7 @@ private enum Metric {
         SCRIPT("script"),
         DISCOVERY("discovery"),
         INGEST("ingest"),
-        ADAPTIVE_SELECTION("adaptiveSelection");
+        ADAPTIVE_SELECTION("adaptive_selection");
 
         private String metricName;