📝 document queued sink methods

docs/api/queued-sink-pending-count.md

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+---
+name: queued-sink-pending-count
+group: api
+category: sink
+update-time: 20260613
+description: Read the current queued-record count from a QueuedSink.
+key-word:
+    - sink
+    - queue
+    - backlog
+    - public
+---
+
+## Queued-sink-pending-count
+
+Read the current queued-record count from a `QueuedSink[S]`. This is the direct sink-level backlog metric for records that have been queued but not yet drained to the wrapped sink.
+
+### Interface
+
+```moonbit
+pub fn[S] QueuedSink::pending_count(self : QueuedSink[S]) -> Int {
+```
+
+#### input
+
+- `self : QueuedSink[S]` - Queued sink whose current pending backlog should be inspected.
+
+#### output
+
+- `Int` - Current number of queued records.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The return value is `self.queue.length()` at the time of the call.
+- This is a point-in-time metric and may change immediately after it is read.
+- It reflects queued records that have not yet been drained or flushed to the wrapped sink.
+- This helper does not mutate the sink.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Direct Queue Backlog Visibility
+
+When code is working with a `QueuedSink` value directly and wants to inspect backlog pressure:
+```moonbit
+let pending = sink.pending_count()
+```
+
+In this example, callers can inspect the current queue size without draining it.
+
+#### When Verify Drain Progress
+
+When manual draining should be checked operationally:
+```moonbit
+ignore(sink.drain(max_items=8))
+ignore(sink.pending_count())
+```
+
+In this example, the metric helps verify whether a drain step reduced queued backlog.
+
+### Error Case
+
+e.g.:
+- This helper does not have a normal failure mode; it only reads current queue length.
+
+- If callers only need simple threshold buffering instead of an explicit queue, `BufferedSink::pending_count()` may be the better API.
+
+### Notes
+
+1. Use this helper for direct visibility into queue backlog on `QueuedSink` values.
+
+2. Pair it with `dropped_count()` when investigating queue pressure.