Backport(v1.19): test_in_monitor_agent: ensure to shutdown plugin instances (#5323)

[github-actions]

Which issue(s) this PR fixes:
Backport #5323
Fixes #

What this PR does / why we need it:
Fix thread and resource leaks in test_in_monitor_agent.rb by ensuring proper shutdown sequences for plugin instances.

Currently, the test cases for in_monitor_agent start plugin instances but do not properly shut them down at the end. Because this plugin utilizes helpers (such as http_server and timer), bypassing the proper stop/shutdown lifecycle leaves their background threads running indefinitely.

As a result, executing the test file leaves many orphaned threads, which could lead to test hangs or resource exhaustion in CI environments.

$ ruby -Ilib:test -e "at_exit { puts '--- Thread count at exit: ' + Thread.list.size.to_s;  pp Thread.list }; require './test/plugin/test_in_monitor_agent.rb'"
...
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
24 tests, 42 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
5.09 tests/s, 8.90 assertions/s
--- Thread count at exit: 34
[#<Thread:0x00007f35334b7f48 run>,
 #<Thread:0x00007f3516211f90@event_loop /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f351620cd10@in_monitor_http_server_helper /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f3516e1e9c8@event_loop /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f3516e1a300@in_monitor_http_server_helper /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f3516e74620@flush_thread_0 /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f3516e730b8@enqueue_thread /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f351705df40@event_loop /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f351713fc88@in_monitor_http_server_helper /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f351409ab78@event_loop /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f3514099160@in_monitor_http_server_helper /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f35140bdb00@Timeout stdlib thread /home/watson/.rbenv/versions/4.0.2/lib/ruby/4.0.0/timeout.rb:87 sleep_forever>,
 #<Thread:0x00007f3516e18de8@event_loop /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f351623b7c8@in_monitor_http_server_helper /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f3516076730@event_loop /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f3516072b08@in_monitor_http_server_helper /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f3514059c18@event_loop /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f35140566d0@in_monitor_http_server_helper /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f35162ce848@event_loop /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f35162c7cc8@in_monitor_http_server_helper /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f35160789b8@event_loop /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f35160766b8@in_monitor_http_server_helper /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f35172c1020@event_loop /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f35172e63e8@in_monitor_http_server_helper /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f34f792ef40@event_loop /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f34f792d230@in_monitor_http_server_helper /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f35172cc4e8@event_loop /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f35172c0350@in_monitor_http_server_helper /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f34f7ad76a8@event_loop /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f34f7ad53f8@in_monitor_http_server_helper /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f35160808e8@event_loop /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f351621c530@in_monitor_http_server_helper /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f34f7902918@event_loop /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>,
 #<Thread:0x00007f34f796e848@in_monitor_http_server_helper /home/watson/src/fluentd/lib/fluent/plugin_helper/thread.rb:70 sleep>]

Docs Changes:
N/A

Release Note:
N/A