in_tail: Add debug log for pos file compaction

[daipom]

Which issue(s) this PR fixes:

• Related to Logs are missing when new file is rotated in tail plugin #4212

What this PR does / why we need it:
Add debug logs about pos file compaction like the following.

[debug]: ... Compacted entries: [...]
[debug]: ... Remove missing entries. existing_targets=[...] entries_after_removing=[...]

Example on the startup.

2023-07-06 22:55:49 +0900 [info]: #0 fluent/log.rb:360:info: starting fluentd worker pid=1347502 ppid=1347482 worker=0
2023-07-06 22:55:49 +0900 [debug]: #0 fluent/log.rb:339:debug: Compacted entries: [101581096, 101581105]
2023-07-06 22:55:49 +0900 [debug]: #0 fluent/log.rb:339:debug: Remove missing entries. existing_targets=[101581096, 101581105] entries_after_removing=[101581096, 101581105]
2023-07-06 22:55:49 +0900 [debug]: #0 fluent/log.rb:339:debug: tailing paths: target = /test/fluentd/input4/test.log,/test/fluentd/input4/test.log.1 | existing = 
2023-07-06 22:55:49 +0900 [info]: #0 fluent/log.rb:360:info: following tail of /test/fluentd/input4/test.log
2023-07-06 22:55:49 +0900 [info]: #0 fluent/log.rb:360:info: following tail of /test/fluentd/input4/test.log.1
2023-07-06 22:55:49 +0900 [info]: #0 fluent/log.rb:360:info: fluentd worker is now running worker=0

In discussion #4212, we are in trouble because we can't know the status of pos file on startup.
We need such logs to know if in_tail reads the pos file and if in_tail finds out the existing target files as expected at startup.

Docs Changes:
No need.

Release Note:
Same as the title.

[ashie]

👍

[daipom]

Thanks for your review!