Reload race condition causes persistent 500 responses on `/aggregated_metrics` endpoint

### Describe the bug

When `fluentd` receives `SIGHUP` (e.g. via `systemctl reload fluentd.service`) to reload its
configuration, worker threads restart in-place. A new worker thread hits `Errno::EADDRINUSE`
when trying to bind its port because the old thread has not yet fully released the socket. The
crashing thread terminates silently, leaving the `fluent-plugin-prometheus` HTTP server on port
24231 running but unable to collect stats from the dead thread. All subsequent requests to the
`/aggregated_metrics` endpoint return HTTP 500 with body
`"Connection refused - connect(2) for 127.0.0.1:"`.

### To Reproduce

1. Install `fluent-package` with the following configuration (see **Your Configuration** below)
2. Start Fluentd: `systemctl start fluentd`
3. Send a reload signal: `systemctl reload fluentd.service` (sends SIGHUP)
4. Observe the warn log for `EADDRINUSE` on one of the worker threads
5. Request `http://localhost:24231/aggregated_metrics` — it returns HTTP 500 with body
`"Connection refused - connect(2) for 127.0.0.1:"`

### Expected behavior

After a reload (SIGHUP), all worker threads should successfully rebind their ports and Fluentd should continue serving `/aggregated_metrics` with HTTP 200. If a thread cannot rebind, it should either retry or fall back gracefully rather than crashing and leaving the supervisor in a broken state.

### Your Environment

```markdown
- Fluentd version: fluentd 1.19.1 (efdc4dca81c23480c9b55e13e55de6aa925b1cf5)
- Package version: fluent-package 6.0.1
- Operating system: Ubuntu 24.04.3
- Kernel version: 6.8.0-94-generic
```

### Your Configuration

```apache

workers 4

@type prometheus
bind 0.0.0.0
port 24231
metrics_path /metrics

@type prometheus_output_monitor

@type prometheus_monitor

@type http
bind 127.0.0.1
port 24224

@type json

@type null

```

### Your Error Log

```shell
2026-03-31 22:47:09 +0000 [warn]: #3 0.07s: Async::Task
| Task may have ended with unhandled exception.
| Errno::EADDRINUSE: Address already in use - bind
| → /opt/fluent/lib/ruby/gems/3.4.0/gems/io-endpoint-0.15.2/lib/io/endpoint/wrapper.rb:152 in 'Socket#bind'
| /opt/fluent/lib/ruby/gems/3.4.0/gems/io-endpoint-0.15.2/lib/io/endpoint/wrapper.rb:152 in 'IO::Endpoint::Wrapper#bind'
| /opt/fluent/lib/ruby/gems/3.4.0/gems/io-endpoint-0.15.2/lib/io/endpoint/host_endpoint.rb:68 in 'block in IO::Endpoint::HostEndpoint#bind'
| /opt/fluent/lib/ruby/gems/3.4.0/gems/io-endpoint-0.15.2/lib/io/endpoint/host_endpoint.rb:67 in 'Array#each'
| /opt/fluent/lib/ruby/gems/3.4.0/gems/io-endpoint-0.15.2/lib/io/endpoint/host_endpoint.rb:67 in 'Enumerator#each'
| /opt/fluent/lib/ruby/gems/3.4.0/gems/io-endpoint-0.15.2/lib/io/endpoint/host_endpoint.rb:67 in 'Enumerable#map'
| /opt/fluent/lib/ruby/gems/3.4.0/gems/io-endpoint-0.15.2/lib/io/endpoint/host_endpoint.rb:67 in 'IO::Endpoint::HostEndpoint#bind'
| /opt/fluent/lib/ruby/gems/3.4.0/gems/async-http-0.89.0/lib/async/http/endpoint.rb:216 in 'Async::HTTP::Endpoint#bind'
| /opt/fluent/lib/ruby/gems/3.4.0/gems/io-endpoint-0.15.2/lib/io/endpoint/generic.rb:82 in 'IO::Endpoint::Generic#accept'
| /opt/fluent/lib/ruby/gems/3.4.0/gems/async-http-0.89.0/lib/async/http/server.rb:67 in 'block in Async::HTTP::Server#run'
| /opt/fluent/lib/ruby/gems/3.4.0/gems/async-2.24.0/lib/async/task.rb:200 in 'block in Async::Task#run'
| /opt/fluent/lib/ruby/gems/3.4.0/gems/async-2.24.0/lib/async/task.rb:438 in 'block in Async::Task#schedule'
```

### Additional context

The crash leaves the `fluent-plugin-prometheus` `/aggregated_metrics` endpoint permanently
returning HTTP 500 (body: `"Connection refused - connect(2) for 127.0.0.1:"`) until
Fluentd is fully restarted. The affected thread is one of the per-worker Prometheus stats
collectors that the aggregation endpoint queries internally.

The stack trace points to `io-endpoint-0.15.2` and `async-http-0.89.0` — the new thread starts
its HTTP server bind before the old thread's socket is fully closed, suggesting either a missing
`SO_REUSEPORT`/`SO_REUSEADDR` option or insufficient drain time before rebinding during a
SIGHUP-triggered reload.

Our workaround is to avoid `systemctl reload` (SIGHUP) in favor of `systemctl restart` (full
stop + start), which guarantees the old process is dead and all sockets released before the new
one starts.

Reproduce the SIGHUP reload with the supplied four-worker configuration, then inspect the worker shutdown and rebind path alongside the io-endpoint-0.15.2 and async-http-0.89.0 stack frames. Verify the fix by confirming no EADDRINUSE warning occurs and /aggregated_metrics continues returning HTTP 200 after reload.

Written by the indexing model from the issue text.