child_process: Plug file descriptor leak

Pipes for child processes aren't closed explicitly after exiting child
processes. Should't rely on GC, close immediately as soon as it isn't
needed to avoid exhausting file descriptors.

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

lib/fluent/plugin_helper/child_process.rb

@@ -346,6 +346,9 @@ def child_process_execute_once(
           if cb
             cb.call(process_info.exit_status) rescue nil
           end
+          process_info.readio&.close rescue nil
+          process_info.writeio&.close rescue nil
+          process_info.stderrio&.close rescue nil
         end
         thread[:_fluentd_plugin_helper_child_process_running] = true
         thread[:_fluentd_plugin_helper_child_process_pid] = pid