feat(log): wire log db, metrics, and body toggle

internal/service/logger.go

@@ -2,6 +2,7 @@ package service
 
 import (
 	"context"
+	"expvar"
 	"log/slog"
 	"time"
 
@@ -9,6 +10,11 @@ import (
 	"gorm.io/gorm"
 )
 
+var (
+	logBatchWriteTotal = expvar.NewInt("log_write_batch_total")
+	logQueueDropped    = expvar.NewInt("log_queue_dropped_total")
+)
+
 // LogWriter batches log records to reduce IO overhead.
 type LogWriter struct {
 	ch            chan model.LogRecord
@@ -48,6 +54,8 @@ func (w *LogWriter) Start(ctx context.Context) {
 			}
 			if err := w.db.Create(&buf).Error; err != nil {
 				slog.Default().Error("log batch insert failed", "err", err)
+			} else {
+				logBatchWriteTotal.Add(1)
 			}
 			buf = buf[:0]
 		}
@@ -75,5 +83,6 @@ func (w *LogWriter) Write(rec model.LogRecord) {
 	case w.ch <- rec:
 	default:
 		// drop to avoid blocking hot path
+		logQueueDropped.Add(1)
 	}
 }