fix: make random string unique

[y1yang0]

Fix the following compilation error:

 # github.com/go-kratos/kratos/v2/transport/grpc
        [instrument] 2024/11/15 13:54:28 Apply func rule github.com/go-kratos/kratos/v2/transport/grpc@[2.5.2,2.8.3)@DialInsecure {KratosDialInsecureOnEnter KratosDialInsecureOnExit}
        [instrument] 2024/11/15 13:54:28 Apply func rule github.com/go-kratos/kratos/v2/transport/grpc@[2.5.2,2.8.3)@WithMiddleware {KratosGRPCWithMiddlewareOnEnter }
        [instrument] 2024/11/15 13:54:29 Apply func rule github.com/go-kratos/kratos/v2/transport/grpc@[2.5.2,2.8.3)@NewServer {KratosNewGRPCServiceOnEnter }
        [instrument] 2024/11/15 13:54:29 RunCmd: github.com/go-kratos/kratos/v2/transport/grpc (281.134908ms)
        # kratosTest/v2.7.2/otel_rules
        otel_rules/otel_setup_inst.go:27:8: http369 redeclared in this block
            otel_rules/otel_setup_inst.go:13:8: other declaration of http369
        otel_rules/otel_setup_inst.go:27:8: "net/http" imported as http369 and not used
        otel_rules/otel_setup_inst.go:93:10: undefined: http369.ClientOnEnterImpl
        otel_rules/otel_setup_inst.go:94:10: undefined: http369.ClientOnExitImpl
        otel_rules/otel_setup_inst.go:97:10: undefined: http369.ServerOnEnterImpl
        otel_rules/otel_setup_inst.go:98:10: undefined: http369.ServerOnExitImpl

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.58%. Comparing base (cd10adf) to head (308451f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #188 +/- ## ======================================= Coverage 83.58% 83.58% ======================================= Files 19 19 Lines 780 780 ======================================= Hits 652 652 Misses 102 102 Partials 26 26 ```

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