📝 document file sink counters and resets

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

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+---
+name: file-sink-rotation-failures
+group: api
+category: sink
+update-time: 20260613
+description: Read the number of rotation failures recorded by a FileSink.
+key-word:
+    - file
+    - sink
+    - rotation
+    - public
+---
+
+## File-sink-rotation-failures
+
+Read the number of rotation failures recorded by a `FileSink`. This helper is useful when direct runtime rotation behavior should be observed operationally.
+
+### Interface
+
+```moonbit
+pub fn FileSink::rotation_failures(self : FileSink) -> Int {
+```
+
+#### input
+
+- `self : FileSink` - File sink whose rotation-failure counter should be inspected.
+
+#### output
+
+- `Int` - Number of recorded rotation failures.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The sink reports its recorded rotation-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 Rotation-specific Diagnostics
+
+When support output should reveal whether direct sink rotation is failing:
+```moonbit
+let count = sink.rotation_failures()
+```
+
+In this example, the concrete file sink exposes a focused metric for rotation-path health.
+
+#### When Validate Runtime Rotation Tuning
+
+When changed rotation settings should be checked in operation:
+```moonbit
+ignore(sink.rotation_failures())
+```
+
+In this example, callers can observe whether runtime rotation changes introduced problems.
+
+### Error Case
+
+e.g.:
+- If callers need both rotation config and failure metrics, they should combine this helper with `rotation_config()` or `state()`.
+
+- This counter alone does not say whether the sink is currently available.
+
+### Notes
+
+1. Use this helper when diagnosing direct file rotation reliability.
+
+2. It is especially relevant when runtime rotation policy can change after startup.