Repository: etcd-io/etcd (language: go)

Issue:
Enabling authentication causes noisy logs for every /readyz call
### Bug report criteria

- [X] This bug report is not security related, security issues should be disclosed privately via [etcd maintainers](mailto:etcd-maintainers@googlegroups.com).
- [X] This is not a support request or question, support requests or questions should be raised in the etcd [discussion forums](https://github.com/etcd-io/etcd/discussions).
- [X] You have read the etcd [bug reporting guidelines](https://github.com/etcd-io/etcd/blob/main/Documentation/contributor-guide/reporting_bugs.md).
- [X] Existing open issues along with etcd [frequently asked questions](https://etcd.io/docs/latest/faq) have been checked and this is not a duplicate.

### What happened?

We run etcd with authentication enabled, we have a service discovery system that frequently polls the `/readyz` HTTP endpoint on etcd server. For every `/readyz` request, etcd prints two log statements  saying `"deleted a simple token"`. Example output:

```
{"level":"info","ts":"2024-06-28T11:55:48.638643-0700",
  "caller":"auth/simple_token.go:170",
  "msg":"deleted a simple token",
  "user-name":"root","token":"swUCgfCSXzbNbVvM.0"}
...
```

Combined with the fact that our service discovery system polls this endpoint every 5 seconds, practically this statement single handedly emits most of the logs we get out of etcd.

### What did you expect to happen?

No logs at the default level for something inconsequential like this.

### How can we reproduce it (as minimally and precisely as possible)?

1. Run etcd server on a macOS terminal

       etcd --auth-token-ttl=5

2. In another terminal window, add a user, then enable authentication

       etcdctl user add root:root
       etcdctl auth enable

3. Make a few successive /readyz queries back to back

       curl 127.0.0.1:2379/readyz 

4. Wait 5 seconds, and observe logs are flooded with this:

		{"level":"info","ts":"2024-06-28T11:55:48.638643-0700","caller":"auth/simple_token.go:170","msg":"deleted a simple token","user-name":"root","token":"swUCgfCSXzbNbVvM.0"}
		{"level":"info","ts":"2024-06-28T11:55:48.638762-0700","caller":"auth/simple_token.go:170","msg":"deleted a simple token","user-name":"root","token":"HbaByRCspoGGJDSQ.0"}
		{"level":"info","ts":"2024-06-28T11:55:50.638493-0700","caller":"auth/simple_token.go:170","msg":"deleted a simple token","user-name":"root","token":"JHPWIPccneJuHHVo.0"}
		{"level":"info","ts":"2024-06-28T11:55:50.638675-0700","caller":"auth/simple_token.go:170","msg":"deleted a simple token","user-name":"root","token":"bPwzKSxHgcHAqDWY.0"}
		{"level":"info","ts":"2024-06-28T11:55:50.638696-0700","caller":"auth/simple_token.go:170","msg":"deleted a simple token","user-name":"root","token":"vHLnwqtzzYivgdjW.0"}
		{"level":"info","ts":"2024-06-28T11:55:50.638828-0700","caller":"auth/simple_token.go:170","msg":"deleted a simple token","user-name":"root","token":"JorAHnUcTharTcGT.0"}
		{"level":"info","ts":"2024-06-28T11:55:50.638845-0700","caller":"auth/simple_token.go:170","msg":"deleted a simple token","user-name":"root","token":"fEcjpFDKzbpmdymM.0"}
		{"level":"info","ts":"2024-06-28T11:55:50.638861-0700","caller":"auth/simple_token.go:170","msg":"deleted a simple token","user-name":"root","token":"nqjiEIoiBHwIMEIz.0"}
		{"level":"info","ts":"2024-06-28T11:55:50.638875-0700","caller":"auth/simple_token.go:170","msg":"deleted a simple token","user-name":"root","token":"AdmAVtAzjwEdYnmV.0"}
		{"level":"info","ts":"2024-06-28T11:55:50.638888-0700","caller":"auth/simple_token.go:170","msg":"deleted a simple token","user-name":"root","token":"lzlBlKmLtPrTxzrh.0"}


### Anything else we need to know?

_No response_

### Etcd version (please run commands below)

<details>

```console
$ etcd --version
etcd Version: 3.5.13
Git SHA: c9063a0dc
Go Version: go1.22.1
Go OS/Arch: darwin/arm64

$ etcdctl version
etcdctl version: 3.5.13
API version: 3.5
```

</details>


### Etcd configuration (command line flags or environment variables)

```
etcd --auth-token-ttl=5
```
(shortened the TTL to illustrate the problem).

### Etcd debug information (please run commands below, feel free to obfuscate the IP address or FQDN in the output)

Not applicable.

### Relevant log output

_No response_


Discussion / hints:

Fix patch:
diff --git a/server/auth/simple_token.go b/server/auth/simple_token.go
index fb9485b4ff9..f8272b185d9 100644
--- a/server/auth/simple_token.go
+++ b/server/auth/simple_token.go
@@ -168,7 +168,7 @@ func (t *tokenSimple) enable() {
 
 	delf := func(tk string) {
 		if username, ok := t.simpleTokens[tk]; ok {
-			t.lg.Info(
+			t.lg.Debug(
 				"deleted a simple token",
 				zap.String("user-name", username),
 				zap.String("token", tk),


Test patch: