Add --json-log flag to enable structured JSON logging (#19968)

Co-authored-by: github_username <github_email>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sgl-model-gateway/bindings/python/src/sglang_router/router_args.py

@@ -45,6 +45,7 @@ class RouterArgs:
     api_key: Optional[str] = None
     log_dir: Optional[str] = None
     log_level: Optional[str] = None
+    json_log: bool = False
     # Service discovery configuration
     service_discovery: bool = False
     selector: Dict[str, str] = dataclasses.field(default_factory=dict)
@@ -413,6 +414,11 @@ class RouterArgs:
             choices=["debug", "info", "warn", "error"],
             help="Set the logging level. If not specified, defaults to INFO.",
         )
+        logging_group.add_argument(
+            f"--{prefix}json-log",
+            action="store_true",
+            help="Enable structured JSON log output instead of plain text.",
+        )
 
         # Service discovery configuration
         k8s_group.add_argument(