Add automatical time logging.

Add: the server will be able to add a `_time` field to log row, by specify `log_server_time: bool = True` in LoggerConfig. To use exact client time instead of server time, make your time field when submit a log row.

DataLogger.py

@@ -13,13 +13,14 @@ Key Architectural Features:
 - **Lazy Initialization & Automatic Management**: The server process is transparently
   started on the first log call and is gracefully shut down on program exit via
   `atexit`, requiring no manual management from the user.
+- **Automatic Server-Side Timestamping**: Optionally, the server can automatically
+  add a timestamp to each log record, indicating the exact time of its arrival.
 - **Unified Interface**: Log data from any process via the simple and consistent
   `DataLogger.log({"key": "value"})` call.
 - **Asynchronous & Batched**: The server process handles I/O asynchronously from the
   clients, batching rows to minimize disk writes and reduce application latency.
 - **Schema Evolution**: Reuses the robust logic to automatically adapt the Parquet
-  schema if new data fields are introduced, rewriting the file to maintain a
-  consistent structure.
+  schema if new data fields are introduced.
 - **Type Handling**: Natively handles Python primitives, NumPy arrays, and PyTorch
   tensors.
 
@@ -109,6 +110,7 @@ class LoggerConfig:
     flush_interval: float = 1.0
     parquet_compression: str = "snappy"
     allow_schema_rewrite: bool = True
+    log_server_time: bool = False
 
 
 class DataLoggerServer(multiprocessing.Process):
@@ -145,7 +147,7 @@ class DataLoggerServer(multiprocessing.Process):
                     item = self._queue.get(timeout=self._config.flush_interval)
 
                     if isinstance(item, dict):
-                        self._buffer.append(self._normalize_row(item))
+                        self._process_log_item(item)
                     elif item == _FLUSH_COMMAND:
                         self._write_buffer_if_needed(force=True)
                         self._flush_event.set()  # Signal completion
@@ -164,6 +166,15 @@ class DataLoggerServer(multiprocessing.Process):
             print(f"FATAL: DataLogger server process crashed: {e}", flush=True)
             traceback.print_exc()
 
+    def _process_log_item(self, item: Row) -> None:  # <<< NEW: Encapsulated logic
+        """Processes a single log item by timestamping, normalizing, and buffering it."""
+        if self._config.log_server_time and "_time" not in item:
+            # Add the server-side timestamp when the item is dequeued.
+            item["_time"] = datetime.datetime.now()
+
+        normalized_row = self._normalize_row(item)
+        self._buffer.append(normalized_row)
+
     def _write_buffer_if_needed(self, force: bool = False) -> None:
         """
         Determines if the buffer should be written to disk and triggers the write.
@@ -185,7 +196,7 @@ class DataLoggerServer(multiprocessing.Process):
             try:
                 item = self._queue.get_nowait()
                 if isinstance(item, dict):
-                    self._buffer.append(self._normalize_row(item))
+                    self._process_log_item(item)
             except queue.Empty:
                 break
         self._write_buffer_if_needed(force=True)