fix: add var declaration to prevent implicit global race condition

metric_data was assigned without var/let/const, creating an
implicit global variable. Concurrent requests to
/api/v1/metric-data could overwrite each other's data, returning
wrong results. Add var to make it function-scoped.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: k-rister

queries/cdmq/server.js

@@ -1637,7 +1637,7 @@ app.post('/api/v1/metric-data', async (req, res) => {
         error: resp['ret-msg']
       });
     }
-    metric_data = resp['data-sets'][0];
+    var metric_data = resp['data-sets'][0];
 
     var labelCount = metric_data && metric_data.values ? Object.keys(metric_data.values).length : 0;
     var elapsed = Date.now() - reqStart;