[Bugfix] Fix PrometheusStatsLogger crash on re-instantiation and non-daemon thread

Two bugs in observability.py:

1. When _metric_mappings is already populated (e.g. multi-engine or
   hot-reload), __init__ returns early without initializing instance
   attributes (is_running, thread, config, etc.). Any subsequent call
   to shutdown() or __del__() raises AttributeError. Fix: always
   initialize core instance attributes before the early-return guard.

2. The update_stats_loop thread is not created as a daemon thread.
   If shutdown() is never explicitly called, this thread prevents the
   Python process from exiting cleanly. Fix: set daemon=True, and
   guard shutdown() against thread being None.

Signed-off-by: supermario_leo <leo.stack@outlook.com>

Author: SuperMarioYL

ucm/observability.py

@@ -66,12 +66,16 @@ def __init__(self, model_name, worker_id, config_path):
         Load metrics config from YAML file (config_path),
         register metrics using prometheus_client, and start a thread to get updated metrics.
         """
+        # Always initialize instance attributes so that shutdown() and
+        # __del__() work even when we skip metric registration below.
+        self.config = self._load_config(config_path)
+        self.log_interval = self.config.get("log_interval", 10)
+        self.is_running = False
+        self.thread = None
+
         if _metric_mappings:
             logger.warning("Metrics are already registered, skipping re-registration.")
             return
-        # Load metrics config
-        self.config = self._load_config(config_path)
-        self.log_interval = self.config.get("log_interval", 10)
 
         # Set up histogram max length
         histogram_max_length = self.config.get("histogram_max_length", 10000)
@@ -97,9 +101,10 @@ def __init__(self, model_name, worker_id, config_path):
         # Initialize metrics based on config
         self._init_metrics_from_config()
 
-        # Start thread to update metrics
+        # Start daemon thread to update metrics so it won't block
+        # process exit if shutdown() is not explicitly called.
         self.is_running = True
-        self.thread = threading.Thread(target=self.update_stats_loop)
+        self.thread = threading.Thread(target=self.update_stats_loop, daemon=True)
         self.thread.start()
 
     def _register_metrics_by_type(self, metric_type):
@@ -190,7 +195,8 @@ def update_stats_loop(self):
 
     def shutdown(self):
         self.is_running = False
-        self.thread.join()
+        if self.thread is not None:
+            self.thread.join()
 
     def __del__(self):
         try: