do not overwrite fmt-dict message in FluentRecordFormatter

[cpruijsen]

FluentRecordFormatter no longer overwrites fmt-dict message with the unstructured fallback when the fmt dict already defines message. _structuring used to replace that field with the full log text after the fmt dict had already applied specifiers such as %(message).5s. Other fmt keys were not overwritten, which is why only message looked broken (#190). Dict and JSON-object merges still last-write-win.

Decision

Skip the fallback only when the fmt dict has a message key. The other options are to make _add_dic never overwrite existing keys, or to skip whenever data already has message. The README describes message as an additional key for string logs; changing _add_dic would change payload-wins-on-overlap for dict/JSON, and testing data would change exclude_attrs traceback melding. Can switch if format keys should always win.

Reporter expected "message":"mingl" in #190, from "%(message).5s" on "mingle: searching for neighbors". Stdlib % formatting produces that truncation; the fmt dict already did too, before the overwrite.

If a fmt dict contains "message": "%(message)s", message is now the stdlib LogRecord.message (no traceback). Use %(exc_text)s for the traceback, as in the issue's config. Omitting message from the fmt dict is unchanged and still includes traceback in the added message field.

Test plan

• test_custom_fmt_truncates_message: reporter case; fails without the fix, passes with it
• test_custom_fmt_truncates_non_string_message: log.info(42) with %(message).1s (same overwrite on the non-string branch)
• tests/test_handler.py and tests/test_asynchandler.py still pass; full pytest tests 74 passed, 2 skipped
• optional: confirm against a live Fluentd with the reporter's logging.config dict