out_file: add warn message for symlink_path setting

[Shingo-Nakayama]

Which issue(s) this PR fixes:

• Partially fixes out_file:Inconsistent path settings and symlink_path settings. #4490

What this PR does / why we need it:
According to the issue #4490 , if a tag or keys is set in the chunk key and those placeholders do not exist in the symlink_path, multiple chunks to compete for the same symlink and some logs can't be taken from it.
Since this is unintended behavior, I think it is necessary to inform the user that this is a bad setting by the warn message.

If a tag or keys is set in the chunk key and those placeholders do not exist in the symlink_path, the following warning log is displayed.

Ex:

<source>
  @type sample
  sample {"message":"hoge","key1":"A","key2":"B"}
  tag test.hoge
</source>

<source>
  @type sample
  sample {"message":"fuga","key1":"A","key2":"B"}
  tag test.fuga
</source>

<match test.**>
  @type file
  path /tmp/test/${tag}/${key1}/${key2}
 symlink_path /tmp/test/link
  append true

  <buffer time,tag,key1,key2>
    @type file
    path /tmp/test/buffer/tag
    timekey 60 
    timekey_wait 3
  </buffer>
</match>

[warn]: #0 Parameter 'symlink_path: /tmp/test/link' doesn't have tag placeholder. This means multiple chunks are competing for a single symlink_path, so some logs may not be taken from the symlink.
[warn]: #0 Parameter 'symlink_path: /tmp/test/link' doesn't have enough placeholders for keys key1,key2. This means multiple chunks are competing for a single symlink_path, so some logs may not be taken from the symlink.

Docs Changes:
None (may be necessary)

Release Note:
Same with the title

[daipom]

Thanks for this fix!
Could you please fix the following points?

[daipom]

LGTM. Thanks!