Non deterministic failure test on: LogStash::Plugin when validating the plugin version logs a warning if the plugin use the milestone

```
Failures:

1) LogStash::Plugin when validating the plugin version logs a warning if the plugin use the milestone option
Failure/Error: Unable to find matching line from backtrace
# received :debug with unexpected arguments
expected: (/stromae plugin is using the 'milestone' method/)
got: ("closing", {:plugin=>100000, enable_metric=>true, codec=>true, charset=>"UTF-8">, threads=>1, message=>"Hello world!">})
# ./logstash-core/lib/logstash/plugin.rb:74:in `do_close'
# ./logstash-core/lib/logstash/pipeline.rb:413:in `inputworker'
# ./logstash-core/lib/logstash/pipeline.rb:382:in `start_input'

Finished in 16.89 seconds (files took 2.9 seconds to load)
569 examples, 1 failure

Failed examples:

rspec ./logstash-core/spec/logstash/plugin_spec.rb:130 # LogStash::Plugin when validating the plugin version logs a warning if the plugin use the milestone option

Randomized with seed 53170
```

Start with logstash-core/spec/logstash/plugin_spec.rb:130 and run the listed RSpec example, repeating it with randomized seeds to reproduce the failure. Trace the warning expectation alongside the unexpected debug call in logstash-core/lib/logstash/plugin.rb:74. Done means the milestone validation test passes reliably without matching an unrelated debug message.

Written by the indexing model from the issue text.