Fix warning printed when chunk key placeholder not replaced

[dobesv]

This used $1 to print the key name. However, there are no capture groups in CHUNK_KEY_PLACEHOLDER_PATTERN, so $1 would not be set. According to the Ruby docs I looked up, $& can be used to get the part of the string that matched, which I think will give the correct output in this case.

[ganmacs]

there are no capture groups in CHUNK_KEY_PLACEHOLDER_PATTERN, so $1 would not be set.

Right. but I think changing the regex pattern of CHUNK_KEY_PLACEHOLDER_PATTERN from /\$\{[-_.@$a-zA-Z0-9]+\}/ to /\$\{([-_.@$a-zA-Z0-9]+)\}/ is better since we need to capture only the value of tag, not a whole of a tag.

an example using $&

CHUNK_KEY_PLACEHOLDER_PATTERN = /\$\{[-_.@$a-zA-Z0-9]+\}/

rvalue = "${a}"

if rvalue =~ CHUNK_KEY_PLACEHOLDER_PATTERN
  p $&                          # "${a}"
end

an example using /\$\{([-_.@$a-zA-Z0-9]+)\}/ for regex pattern

CHUNK_KEY_PLACEHOLDER_PATTERN = /\$\{([-_.@$a-zA-Z0-9]+)\}/

rvalue = "${a}"

if rvalue =~ CHUNK_KEY_PLACEHOLDER_PATTERN
  p $1                          #=> "a"
end

[dobesv]

Personally I would prefer to see the whole ${a} but it's totally unimportant, TBH. Either would be a tremendous improvement.

[dobesv]

You are welcome to make the change your way and make another PR, I won't be updating this PR.

[ganmacs]

it's not for my preference. A current implementation which does the similar thing returns the value of the tag (https://github.com/fluent/fluentd/pull/2523/files#diff-7a3f304568ba6143aaf2b7e78011d044R749). I think it's important to have consistency with other warnings.