📝 Add async logger composition API docs

docs/api/async-logger-with-timestamp.md

@@ -0,0 +1,77 @@
+---
+name: async-logger-with-timestamp
+group: api
+category: async
+update-time: 20260512
+description: Enable or disable automatic timestamp capture for records emitted by an async logger.
+key-word:
+    - async
+    - logger
+    - timestamp
+    - public
+---
+
+## Async-logger-with-timestamp
+
+Enable or disable automatic timestamp capture on async log emission. This API controls whether `AsyncLogger::log(...)` records the current time before enqueue or leaves the timestamp at `0UL`.
+
+### Interface
+
+```moonbit
+pub fn[S] AsyncLogger::with_timestamp(self : AsyncLogger[S], enabled~ : Bool = true) -> AsyncLogger[S] {}
+```
+
+#### input
+
+- `self : AsyncLogger[S]` - Base async logger whose timestamp behavior should change.
+- `enabled : Bool` - Whether emitted records should capture current time automatically.
+
+#### output
+
+- `AsyncLogger[S]` - A new async logger value with updated timestamp behavior.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- When enabled, `log(...)` captures `@env.now()` before placing the record into the queue.
+- When disabled, emitted records use `0UL` as the timestamp value.
+- This setting affects later emitted records only.
+- Queue, batching, and flush behavior are unchanged.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Real Event Time Before Queueing
+
+When downstream formatting or JSON output should include event time:
+```moonbit
+let logger = async_logger(console_sink())
+  .with_timestamp()
+```
+
+In this example, each record captures its timestamp before entering the async queue.
+
+#### When Need Deterministic Async Records
+
+When timestamps should be disabled for tests or reduced output:
+```moonbit
+let logger = async_logger(console_sink())
+  .with_timestamp(enabled=false)
+```
+
+In this example, queued records are emitted without runtime time capture.
+
+### Error Case
+
+e.g.:
+- If timestamps are disabled, formatters that expect meaningful time values may show empty or zero-like timestamp output.
+
+- If callers need timestamps only for one record, a separate logger value is usually clearer than toggling behavior repeatedly.
+
+### Notes
+
+1. This API controls record creation before enqueue, not formatter display policy.
+
+2. It is useful for tests, deterministic snapshots, and production timing.