enable_input_metrics: inaccurate values

### Describe the bug

We can take metrics of input plugins by setting [enable_input_metrics](https://docs.fluentd.org/deployment/system-config#enable_input_metrics).
However, the value would be inaccurate.

I don't directly confirm it yet, but I have confirmed `@caller_plugin_id` of `EventRouter` has race condition.

* https://github.com/fluent/fluentd/issues/4567#issuecomment-2503380410

So, it would be possible that a wrong `metric_callbacks` is selected.

https://github.com/fluent/fluentd/blob/2d8c9d4b94b1d45e7d4d6e59caae5640a516eabd/lib/fluent/event_router.rb#L96-L102

It would cause wrong metrics calculation.

### To Reproduce

I haven't checked it yet, but the following settings should result in a slight error in the metric values.

```xml

enable_input_metrics

@type monitor_agent

@type sample
tag test.foo
rate 100

@type sample
tag test.bar
rate 100

@type sample
tag test.boo
rate 100

@type null

```

Wait a few minutes and check the metrics.

```bash
curl http://localhost:24220/api/plugins.json | jq
```

```json
{
"plugins": [
{
"plugin_id": "object:d34",
"plugin_category": "input",
"type": "monitor_agent",
"config": {
"@type": "monitor_agent"
},
"output_plugin": false,
"retry_count": null,
"emit_records": 0,
"emit_size": 0
},
{
"plugin_id": "object:d48",
"plugin_category": "input",
"type": "sample",
"config": {
"@type": "sample",
"tag": "test.foo",
"rate": "100"
},
"output_plugin": false,
"retry_count": null,
"emit_records": 43112,
"emit_size": 0
},
{
"plugin_id": "object:d5c",
"plugin_category": "input",
"type": "sample",
"config": {
"@type": "sample",
"tag": "test.bar",
"rate": "100"
},
"output_plugin": false,
"retry_count": null,
"emit_records": 43109,
"emit_size": 0
},
{
"plugin_id": "object:d70",
"plugin_category": "input",
"type": "sample",
"config": {
"@type": "sample",
"tag": "test.boo",
"rate": "100"
},
"output_plugin": false,
"retry_count": null,
"emit_records": 43109,
"emit_size": 0
},
{
"plugin_id": "object:d0c",
"plugin_category": "output",
"type": "null",
"config": {
"@type": "null"
},
"output_plugin": true,
"retry_count": 0,
"emit_records": 129330,
"emit_size": 0,
"emit_count": 129330,
"write_count": 0,
"rollback_count": 0,
"slow_flush_count": 0,
"flush_time_count": 0,
"retry": {}
}
]
}
```

You can confirm a difference in the value of `emit_records` for each `in_sample`.

### Expected behavior

There is no difference in the value of `emit_records` for each `in_sample`.

### Your Environment

```markdown
- Fluentd version: 1.18.0
- Package version:
- Operating system: Ubuntu 20.04.6 LTS (Focal Fossa)
- Kernel version: 5.15.0-124-generic
```

### Your Configuration

```apache
Noted in `To Reproduce`.
```

### Your Error Log

```shell
No error.
```

### Additional context

_No response_

Start with lib/fluent/event_router.rb at lines 96-102 and trace how @caller_plugin_id selects metric_callbacks when enable_input_metrics is enabled. Reproduce the configuration using three sample inputs, monitor_agent, and the plugins API, then compare emit_records across inputs. Done means the input metrics remain accurate and the race condition has regression coverage.

Written by the indexing model from the issue text.