feat(observability): add structured application logging

Emit correlated JSON logs from the API and FunASR services, propagate request IDs to the client, and configure bounded opt-in Docker logging.

Document the host-wide Grafana/Loki query workflow and keep observability deployment ownership outside the application repository.

asr-service/logging_config.py

@@ -0,0 +1,90 @@
+from __future__ import annotations
+
+import json
+import logging
+import os
+import sys
+from datetime import datetime, timezone
+from typing import Any
+
+LOG_FIELDS = (
+    "event",
+    "request_id",
+    "segment_id",
+    "feedback_session_id",
+    "status",
+    "latency_ms",
+    "audio_size_bytes",
+    "audio_duration_ms",
+    "audio_format",
+    "error_class",
+)
+
+
+class JsonFormatter(logging.Formatter):
+    def format(self, record: logging.LogRecord) -> str:
+        payload: dict[str, Any] = {
+            "timestamp": datetime.now(timezone.utc).isoformat(timespec="milliseconds"),
+            "level": record.levelname,
+            "service": "funasr",
+            "environment": os.getenv("APP_ENV", "development"),
+            "app_version": os.getenv("APP_VERSION", "unknown"),
+            "git_sha": os.getenv("GIT_SHA", "unknown"),
+            "message": record.getMessage(),
+        }
+        for field in LOG_FIELDS:
+            value = getattr(record, field, None)
+            if value is not None:
+                payload[field] = value
+        return json.dumps(payload, ensure_ascii=False, default=str)
+
+
+class JsonLogStream:
+    encoding = "utf-8"
+
+    def __init__(self, level: int) -> None:
+        self.level = level
+        self.buffer = ""
+
+    def write(self, message: str) -> int:
+        self.buffer += message
+        while "\n" in self.buffer:
+            line, self.buffer = self.buffer.split("\n", 1)
+            self._emit(line)
+        return len(message)
+
+    def flush(self) -> None:
+        if self.buffer:
+            self._emit(self.buffer)
+            self.buffer = ""
+
+    def isatty(self) -> bool:
+        return False
+
+    def _emit(self, message: str) -> None:
+        message = message.strip()
+        if message:
+            logging.getLogger("funasr-service").log(self.level, message)
+
+
+def configure_logging() -> None:
+    handler = logging.StreamHandler(sys.__stdout__)
+    handler.setFormatter(JsonFormatter())
+    root = logging.getLogger()
+    root.handlers.clear()
+    root.addHandler(handler)
+    root.setLevel(logging.INFO)
+
+    for name, logger in logging.Logger.manager.loggerDict.items():
+        if isinstance(logger, logging.Logger):
+            logger.handlers.clear()
+            logger.propagate = name != "uvicorn.access"
+
+
+def redirect_standard_streams() -> None:
+    sys.stdout = JsonLogStream(logging.INFO)  # type: ignore[assignment]
+    sys.stderr = JsonLogStream(logging.ERROR)  # type: ignore[assignment]
+
+
+def log_event(level: int, event: str, message: str, **fields: Any) -> None:
+    logging.getLogger("funasr-service").log(level, message, extra={"event": event, **fields})