enable_input_metrics: inaccurate values

[daipom]

Describe the bug

We can take metrics of input plugins by setting 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.

• Error event logs should include plugin id #4567 (comment)

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

fluentd/lib/fluent/event_router.rb

Lines 96 to 102 in 2d8c9d4

	def find_callback
	if @caller_plugin_id
	@metric_callbacks[@caller_plugin_id]
	else
	nil
	end
	end

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.

<system>
  enable_input_metrics
</system>

<source>
  @type monitor_agent
</source>

<source>
  @type sample
  tag test.foo
  rate 100
</source>

<source>
  @type sample
  tag test.bar
  rate 100
</source>

<source>
  @type sample
  tag test.boo
  rate 100
</source>

<match test.**>
  @type null
</match>

Wait a few minutes and check the metrics.

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

{
  "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

- 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

Noted in `To Reproduce`.

Your Error Log

No error.

Additional context

No response