in_tail: Introduce read_interval option

[linxiulei]

fs_stat uses a fixed interval to read watched files. Users may choose higher interval for lower overhead or lower interval for lower latency. This commit allows users to configure it.

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

• Example configuration file for the change

[INPUT]
    Name             tail
    Alias            kube_containers
    Path             /tmp/log/*.log
    Multiline        Off
    Mem_Buf_Limit    5MB
    Buffer_Max_Size  1MB
    Skip_Long_Lines  On
    Refresh_Interval 10
    Read_from_Head   True
    Inotify_Watcher false
    Stat.Read_Interval_Sec 12
    Stat.Read_Interval_Nsec 10

[OUTPUT]
    Name   stdout
    Match  *

• Debug log output from testing the change

valgrind.log

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

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

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

Documentation

• [N/A] Documentation required for this feature

Backporting

• Backport to latest stable release.

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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.

[github-actions]

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[linxiulei]

Can someone review this? Thanks!