📝 document record transformation apis

docs/api/record-with-message.md

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+---
+name: record-with-message
+group: api
+category: record
+update-time: 20260613
+description: Return a copy of a record with a replaced message field.
+key-word:
+    - record
+    - message
+    - transform
+    - public
+---
+
+## Record-with-message
+
+Return a copy of a `Record` with a different `message`. This helper is the narrowest way to rewrite the visible event text while preserving the rest of the structured event data.
+
+### Interface
+
+```moonbit
+pub fn Record::with_message(self : Record, message : String) -> Record {}
+```
+
+#### input
+
+- `self : Record` - Base record whose message should be replaced.
+- `message : String` - New message text for the returned record.
+
+#### output
+
+- `Record` - New record value carrying the updated message.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This helper returns a new record value instead of mutating the original one.
+- Only the `message` field is replaced.
+- `level`, `timestamp_ms`, `target`, and `fields` are preserved.
+- This is useful for adapters, tests, or explicit value-level rewriting before formatting or sink dispatch.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need To Rewrite One Event Message
+
+When one record should keep its metadata but use different visible text:
+```moonbit
+let rec = Record::new(Level::Error, "raw failure", target="bridge")
+let next = rec.with_message("dispatch failed")
+```
+
+In this example, the new message is applied without rebuilding the rest of the record manually.
+
+#### When Need To Normalize Message Text In Tests
+
+When one test record should be adapted to a different wording:
+```moonbit
+let rec = Record::new(Level::Info, "ok").with_message("started")
+```
+
+In this example, the helper keeps the change focused on one field.
+
+### Error Case
+
+e.g.:
+- If `message` is empty, the returned record is still valid and simply carries an empty message string.
+
+- If the same rewrite should be reusable across many records, `prefix_message(...)` or a custom `RecordPatch` may be a better fit.
+
+### Notes
+
+1. Use this helper for one-off value-level message replacement.
+
+2. Use patch helpers when message rewriting should participate in a reusable logger pipeline.