fix(metrics): publish failure metrics for SQL analytics engine path

The SQL analytics router in SQLPlugin handled errors by sending a raw
BytesRestResponse without incrementing FAILED_REQ_COUNT_CUS or
FAILED_REQ_COUNT_SYS, so SQL+AE failures were invisible in stats. Unlike
the PPL path, the router owns the channel and never routes failures back
to RestSqlAction's terminal error handler.

Make RestSqlAction.handleException public static and call it from the
analytics router onFailure callbacks so failure metrics and proper error
formatting are applied consistently. The PPL+AE path already publishes
metrics via RestPPLQueryAction's listener, so no change is needed there.

Signed-off-by: Chen Dai <daichen@amazon.com>

Author: dai-chen

legacy/src/main/java/org/opensearch/sql/legacy/plugin/RestSqlAction.java

@@ -197,14 +197,14 @@ private void delegateToV2Engine(
                   handleException(restChannel, e);
                 }
               },
-              this::handleException)
+              RestSqlAction::handleException)
           .accept(channel);
     } catch (Exception e) {
       handleException(channel, e);
     }
   }
 
-  private void handleException(RestChannel restChannel, Exception exception) {
+  public static void handleException(RestChannel restChannel, Exception exception) {
     RestStatus status = getRestStatus(exception);
     logAndPublishMetrics(status, exception);
     reportError(restChannel, exception, status);
@@ -337,12 +337,13 @@ private static boolean isClientError(Exception e) {
         || e instanceof ExpressionEvaluationException;
   }
 
-  private void sendResponse(
+  private static void sendResponse(
       final RestChannel channel, final String message, final RestStatus status) {
     channel.sendResponse(new BytesRestResponse(status, message));
   }
 
-  private void reportError(final RestChannel channel, final Exception e, final RestStatus status) {
+  private static void reportError(
+      final RestChannel channel, final Exception e, final RestStatus status) {
     sendResponse(
         channel, ErrorMessageFactory.createErrorMessage(e, status.getStatus()).toString(), status);
   }

plugin/src/main/java/org/opensearch/sql/plugin/SQLPlugin.java

@@ -263,8 +263,7 @@ protected Object buildJsonObject(ExplainResponse resp) {
 
               @Override
               public void onFailure(Exception e) {
-                channel.sendResponse(
-                    new BytesRestResponse(RestSqlAction.getRestStatus(e), e.getMessage()));
+                RestSqlAction.handleException(channel, e);
               }
             });
       } else {
@@ -282,8 +281,7 @@ public void onResponse(TransportPPLQueryResponse response) {
 
               @Override
               public void onFailure(Exception e) {
-                channel.sendResponse(
-                    new BytesRestResponse(RestSqlAction.getRestStatus(e), e.getMessage()));
+                RestSqlAction.handleException(channel, e);
               }
             });
       }