📝 document file sink counters and resets

docs/api/file-sink-open-failures.md

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+---
+name: file-sink-open-failures
+group: api
+category: sink
+update-time: 20260613
+description: Read the number of open failures recorded by a FileSink.
+key-word:
+    - file
+    - sink
+    - open
+    - public
+---
+
+## File-sink-open-failures
+
+Read the number of open failures recorded by a `FileSink`. This helper is useful for diagnosing direct file-availability or reopen problems.
+
+### Interface
+
+```moonbit
+pub fn FileSink::open_failures(self : FileSink) -> Int {
+```
+
+#### input
+
+- `self : FileSink` - File sink whose open-failure counter should be inspected.
+
+#### output
+
+- `Int` - Number of recorded open failures.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The sink reports its recorded open-failure count directly.
+- The counter is cumulative until reset.
+- This helper is observation-only and does not mutate sink state.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need File Availability Diagnostics
+
+When operators should see whether file open or reopen failed:
+```moonbit
+let count = sink.open_failures()
+```
+
+In this example, the direct sink exposes the open-failure metric directly.
+
+#### When Validate Recovery Logic
+
+When tests or support code should inspect whether reopen attempts failed:
+```moonbit
+ignore(sink.open_failures())
+```
+
+In this example, the counter helps confirm whether file-open problems occurred during runtime.
+
+### Error Case
+
+e.g.:
+- If callers need the full file status snapshot rather than one counter, `state()` is the better API.
+
+- This counter does not tell you which particular reopen attempt failed.
+
+### Notes
+
+1. Use this helper for focused direct file-open diagnostics.
+
+2. Pair it with `reopen(...)` and `is_available()` during recovery analysis.