Pass chunk directly instead of chunk metadata

[cosmo0920]

extract_placeholders should be passed chunk instance directly instead of chunk.metadata.

[y-ken]

Thanks for this fix, @cosmo0920 — and apologies it sat unreviewed for so long. 🙏

This is correct. In current Fluentd, extract_placeholders(str, chunk) expects the chunk itself; passing chunk.metadata only works through the backward-compat path, which the source explicitly marks as legacy:

# fluentd lib/fluent/plugin/output.rb
def extract_placeholders(str, chunk)
  metadata = if chunk.is_a?(Fluent::Plugin::Buffer::Chunk)
               chunk_passed = true
               chunk.metadata
             else
               chunk_passed = false
               # For existing plugins. Old plugin passes Chunk.metadata instead of Chunk
               chunk
             end
  ...

Besides aligning with the current API, passing the chunk also enables the ${chunk_id} placeholder in database/table — the metadata path logs "${chunk_id} is not allowed in this plugin. Pass Chunk instead of metadata in extract_placeholders's 2nd argument".

I applied this on top of the current code and ran the full suite (including new real-connection integration tests) on Ruby 3.4 / Fluentd 1.19.2 against MySQL 8.4 — all green (18 tests, 63 assertions, 0 failures). The fluentd >= 0.14.22 bump is reasonable as well.

LGTM 👍 I'd like to merge this once the new GitHub Actions CI (#72) lands so it runs through CI; a quick rebase on top of that would be ideal. Thanks again for the fix!

[y-ken]

@tagomoris @toyama0919 I'll organize some of the pending pull requests and issues.