in_tail: process non utf8 encodings with conversion engine

[cosmo0920]

This PR is a subsequent PR for #10464.
For a long time, we just processed UTF-8 and recently adding UTF-16 series of character encoding set.
But, in the real world, we need to handle more encoding set such as GBK, ShiftJIS, GB18030, BIG5, UHC and other europian and middle east characters.

In this PR, I added a handler to process such encodings in in_tail plugin.
With this PR, we'll start to process these encodings with generic.encoding.

---

Enter [N/A] in the box, if an item is not applicable to your change.

Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change

[INPUT]
    Name         tail
    Tag          dummy.local
    Path         /path/to/fluent-bit/tests/runtime/data/tail/log/generic_enc_gbk.log
    generic.encoding GBK
    Read_From_Head true
[OUTPUT]
    Name  stdout
    Match *

• Debug log output from testing the change

Fluent Bit v4.0.4
* Copyright (C) 2015-2025 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

______ _                  _    ______ _ _             ___  _____ 
|  ___| |                | |   | ___ (_) |           /   ||  _  |
| |_  | |_   _  ___ _ __ | |_  | |_/ /_| |_  __   __/ /| || |/' |
|  _| | | | | |/ _ \ '_ \| __| | ___ \ | __| \ \ / / /_| ||  /| |
| |   | | |_| |  __/ | | | |_  | |_/ / | |_   \ V /\___  |\ |_/ /
\_|   |_|\__,_|\___|_| |_|\__| \____/|_|\__|   \_/     |_(_)___/ 


[2025/07/04 18:40:09] [ info] Configuration:
[2025/07/04 18:40:09] [ info]  flush time     | 1.000000 seconds
[2025/07/04 18:40:09] [ info]  grace          | 5 seconds
[2025/07/04 18:40:09] [ info]  daemon         | 0
[2025/07/04 18:40:09] [ info] ___________
[2025/07/04 18:40:09] [ info]  inputs:
[2025/07/04 18:40:09] [ info]      tail
[2025/07/04 18:40:09] [ info] ___________
[2025/07/04 18:40:09] [ info]  filters:
[2025/07/04 18:40:10] [ info] ___________
[2025/07/04 18:40:10] [ info]  outputs:
[2025/07/04 18:40:10] [ info]      stdout.0
[2025/07/04 18:40:10] [ info] ___________
[2025/07/04 18:40:10] [ info]  collectors:
[2025/07/04 18:40:10] [ info] [fluent bit] version=4.0.4, commit=908b11d7f1, pid=1515015
[2025/07/04 18:40:10] [debug] [engine] coroutine stack size: 24576 bytes (24.0K)
[2025/07/04 18:40:10] [ info] [storage] ver=1.1.6, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2025/07/04 18:40:10] [ info] [simd    ] disabled
[2025/07/04 18:40:10] [ info] [cmetrics] version=1.0.3
[2025/07/04 18:40:10] [ info] [ctraces ] version=0.6.6
[2025/07/04 18:40:10] [ info] [input:tail:tail.0] initializing
[2025/07/04 18:40:10] [ info] [input:tail:tail.0] storage_strategy='memory' (memory only)
[2025/07/04 18:40:10] [debug] [tail:tail.0] created event channels: read=25 write=26
[2025/07/04 18:40:10] [debug] [input:tail:tail.0] flb_tail_fs_inotify_init() initializing inotify tail input
[2025/07/04 18:40:10] [debug] [input:tail:tail.0] inotify watch fd=31
[2025/07/04 18:40:10] [ info] [output:stdout:stdout.0] worker #0 started
[2025/07/04 18:40:10] [debug] [input:tail:tail.0] scanning path /media/Data3/Gitrepo/fluent-bit/tests/runtime/data/tail/log/generic_enc_gbk.log
[2025/07/04 18:40:10] [debug] [input:tail:tail.0] file will be read in POSIX_FADV_DONTNEED mode /media/Data3/Gitrepo/fluent-bit/tests/runtime/data/tail/log/generic_enc_gbk.log
[2025/07/04 18:40:10] [debug] [input:tail:tail.0] inode=43131037 with offset=0 appended as /media/Data3/Gitrepo/fluent-bit/tests/runtime/data/tail/log/generic_enc_gbk.log
[2025/07/04 18:40:10] [debug] [input:tail:tail.0] scan_glob add(): /media/Data3/Gitrepo/fluent-bit/tests/runtime/data/tail/log/generic_enc_gbk.log, inode 43131037
[2025/07/04 18:40:10] [debug] [input:tail:tail.0] 1 new files found on path '/media/Data3/Gitrepo/fluent-bit/tests/runtime/data/tail/log/generic_enc_gbk.log'
[2025/07/04 18:40:10] [debug] [stdout:stdout.0] created event channels: read=33 write=34
[2025/07/04 18:40:10] [ info] [sp] stream processor started
[2025/07/04 18:40:10] [ info] [engine] Shutdown Grace Period=5, Shutdown Input Grace Period=2
[2025/07/04 18:40:10] [debug] [input:tail:tail.0] [static files] processed 46b
[2025/07/04 18:40:10] [debug] [input:tail:tail.0] inode=43131037 file=/media/Data3/Gitrepo/fluent-bit/tests/runtime/data/tail/log/generic_enc_gbk.log promote to TAIL_EVENT
[2025/07/04 18:40:10] [ info] [input:tail:tail.0] inotify_fs_add(): inode=43131037 watch_fd=1 name=/media/Data3/Gitrepo/fluent-bit/tests/runtime/data/tail/log/generic_enc_gbk.log
[2025/07/04 18:40:10] [debug] [input:tail:tail.0] [static files] processed 0b, done
[0] dummy.local: [[1751622010.154549011, {}], {"log"=>"你好"}]
[1] dummy.local: [[1751622010.162688831, {}], {"log"=>"谢谢"}]
[2] dummy.local: [[1751622010.162775758, {}], {"log"=>"再见"}]
[3] dummy.local: [[1751622010.162811553, {}], {"log"=>"中国"}]
[4] dummy.local: [[1751622010.162843340, {}], {"log"=>"猫"}]
[5] dummy.local: [[1751622010.162873918, {}], {"log"=>"狗"}]
[2025/07/04 18:40:10] [debug] [task] created task=0x61694a0 id=0 OK
[6] dummy.local: [[1751622010.162914570, {}], {"log"=>"吃"}]
[7] dummy.local: [[1751622010.162944805, {}], {"log"=>"喝"}]
[2025/07/04 18:40:10] [debug] [output:stdout:stdout.0] task_id=0 assigned to thread #0
[8] dummy.local: [[1751622010.162974608, {}], {"log"=>"天"}]
[9] dummy.local: [[1751622010.163004281, {}], {"log"=>"海"}]
[10] dummy.local: [[1751622010.163034517, {}], {"log"=>"月亮"}]
[11] dummy.local: [[1751622010.163064845, {}], {"log"=>"花"}]
[2025/07/04 18:40:10] [debug] [out flush] cb_destroy coro_id=0
[2025/07/04 18:40:10] [debug] [task] destroy task=0x61694a0 (task_id=0)
^C[2025/07/04 18:40:11] [engine] caught signal (SIGINT)
[2025/07/04 18:40:11] [ warn] [engine] service will shutdown in max 5 seconds
[2025/07/04 18:40:11] [ info] [input] pausing tail.0
[2025/07/04 18:40:11] [ info] [engine] service has stopped (0 pending tasks)
[2025/07/04 18:40:11] [ info] [input] pausing tail.0
[2025/07/04 18:40:11] [ info] [output:stdout:stdout.0] thread worker #0 stopping...
[2025/07/04 18:40:11] [ info] [output:stdout:stdout.0] thread worker #0 stopped
[2025/07/04 18:40:11] [debug] [input:tail:tail.0] inode=43131037 removing file name /media/Data3/

• Attached Valgrind output that shows no leaks or memory corruption was found

==1515015== 
==1515015== HEAP SUMMARY:
==1515015==     in use at exit: 0 bytes in 0 blocks
==1515015==   total heap usage: 3,261 allocs, 3,261 frees, 1,077,833 bytes allocated
==1515015== 
==1515015== All heap blocks were freed -- no leaks are possible
==1515015== 
==1515015== For lists of detected and suppressed errors, rerun with: -s
==1515015== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

fluent/fluent-bit-docs#1870

Backporting

• Backport to latest stable release.

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.