📝 document file sink counters and resets

docs/api/file-sink-reset-failure-counters.md

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+---
+name: file-sink-reset-failure-counters
+group: api
+category: sink
+update-time: 20260613
+description: Reset the file failure counters recorded by a FileSink.
+key-word:
+    - file
+    - sink
+    - reset
+    - public
+---
+
+## File-sink-reset-failure-counters
+
+Reset the file failure counters recorded by a `FileSink`. This helper clears open, write, flush, and rotation failure metrics together on the direct sink.
+
+### Interface
+
+```moonbit
+pub fn FileSink::reset_failure_counters(self : FileSink) -> Unit {
+```
+
+#### input
+
+- `self : FileSink` - File sink whose file failure counters should be cleared.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This helper clears open, write, flush, and rotation failure counters together.
+- It is useful after diagnostics, recovery, or controlled tests.
+- It does not reopen the sink or change file policy.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need A Fresh Diagnostics Baseline
+
+When previous failure history should be cleared before a new observation window:
+```moonbit
+sink.reset_failure_counters()
+```
+
+In this example, future failures can be measured from a clean baseline.
+
+#### When Validate Post-recovery Behavior
+
+When recovery logic should clear old counters before rechecking health:
+```moonbit
+sink.reset_failure_counters()
+ignore(sink.open_failures())
+```
+
+In this example, the direct sink starts a new diagnostics window after reset.
+
+### Error Case
+
+e.g.:
+- If callers need the current values before clearing them, they should read the counters or `state()` first.
+
+- This helper resets metrics only; it does not fix the underlying cause of past failures.
+
+### Notes
+
+1. Use this helper after diagnostics or recovery, not before capturing needed evidence.
+
+2. It is the reset companion for the direct file failure-counter helpers.