feat: expose mysql transaction retry metrics

internal/storage/mysql/db.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"database/sql"
 	"errors"
+	"sync/atomic"
 	"time"
 
 	driver "github.com/go-sql-driver/mysql"
@@ -53,6 +54,24 @@ func Open(ctx context.Context, dsn string, maxOpen, maxIdle int) (*sql.DB, error
 
 const transactionRetryLimit = 2
 
+var transactionRetries atomic.Uint64
+var transactionRetryExhausted atomic.Uint64
+
+// TransactionStats 是本进程提交前事务重试的累计快照。
+// 计数器只记录明确的 MySQL 锁等待超时或死锁回调失败，不记录提交回执不明。
+type TransactionStats struct {
+	Retries   uint64 // 已实际执行的重试次数。
+	Exhausted uint64 // 达到重试上限的事务次数。
+}
+
+// ReadTransactionStats 返回本进程事务重试计数，不访问数据库。
+func ReadTransactionStats() TransactionStats {
+	return TransactionStats{
+		Retries:   transactionRetries.Load(),
+		Exhausted: transactionRetryExhausted.Load(),
+	}
+}
+
 // WithTx retries only a callback failure caused by a MySQL deadlock or lock wait
 // timeout before COMMIT. It never retries Begin/Commit errors because a commit
 // response may be lost after the database has already applied the transaction;
@@ -61,11 +80,15 @@ func WithTx(ctx context.Context, db *sql.DB, fn func(*sql.Tx) error) error {
 	for attempt := 0; attempt <= transactionRetryLimit; attempt++ {
 		err, retryable := withTxAttempt(ctx, db, fn)
 		if err == nil || !retryable || attempt == transactionRetryLimit {
+			if retryable && attempt == transactionRetryLimit {
+				transactionRetryExhausted.Add(1)
+			}
 			return err
 		}
 		if err := waitTransactionRetry(ctx, attempt); err != nil {
 			return err
 		}
+		transactionRetries.Add(1)
 	}
 	return errors.New("transaction retry loop exhausted")
 }