Watcher: Ensure emit_stacktraces parameter works in watcher stats (elastic/x-pack-elasticsearch#2399)

This parameter ceased to work when Elasticsearch 5 introduced strict
parameter handling, because of a missing test.

This commit adds the parameter to the rest handler responseParams()
and adds a test along with the needed YAML definition.

relates elastic/x-pack-elasticsearch#2396

Original commit: elastic/x-pack-elasticsearch@8638df336c

plugin/src/main/java/org/elasticsearch/xpack/watcher/rest/action/RestWatcherStatsAction.java

@@ -15,6 +15,7 @@ import org.elasticsearch.xpack.watcher.rest.WatcherRestHandler;
 import org.elasticsearch.xpack.watcher.transport.actions.stats.WatcherStatsRequest;
 
 import java.io.IOException;
+import java.util.Collections;
 import java.util.Set;
 
 import static org.elasticsearch.rest.RestRequest.Method.GET;
@@ -47,4 +48,13 @@ public class RestWatcherStatsAction extends WatcherRestHandler {
 
         return channel -> client.watcherStats(request, new RestActions.NodesResponseRestListener<>(channel));
     }
+
+    private static final Set<String> RESPONSE_PARAMS = Collections.singleton("emit_stacktraces");
+
+    @Override
+    protected Set<String> responseParams() {
+        // this parameter is only needed when current watches are supposed to be returned
+        // it's used in the WatchExecutionContext.toXContent() method
+        return RESPONSE_PARAMS;
+    }
 }

plugin/src/test/resources/rest-api-spec/api/xpack.watcher.stats.json

@@ -17,6 +17,11 @@
           "type" : "enum",
           "options" : ["_all", "queued_watches", "pending_watches"],
           "description" : "Controls what additional stat metrics should be include in the response"
+        },
+        "emit_stacktraces": {
+          "type" : "boolean",
+          "description" : "Emits stack traces of currently running watches",
+          "required" : false
         }
       }
     },

plugin/src/test/resources/rest-api-spec/test/watcher/stats/10_basic.yml

@@ -4,3 +4,13 @@
   - do: {xpack.watcher.stats: {}}
   - match: { "manually_stopped": false }
   - match: { "stats.0.watcher_state": "started" }
+
+---
+"Test watcher stats supports emit_stacktraces parameter":
+
+  - do:
+      xpack.watcher.stats:
+        metric: "all"
+        emit_stacktraces: "true"
+  - match: { "manually_stopped": false }
+  - match: { "stats.0.watcher_state": "started" }