📝 document sync library facade write apis

docs/api/library-logger-log.md

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+---
+name: library-logger-log
+group: api
+category: facade
+update-time: 20260613
+description: Emit a record through a LibraryLogger facade with explicit level, message, fields, and optional target override.
+key-word:
+    - library
+    - facade
+    - log
+    - public
+---
+
+## Library-logger-log
+
+Emit a record through the library-facing sync logger with an explicit level and message. This is the core write API behind the level-specific `LibraryLogger` convenience methods.
+
+### Interface
+
+```moonbit
+pub fn[S : Sink] LibraryLogger::log(
+  self : LibraryLogger[S],
+  level : Level,
+  message : String,
+  fields~ : Array[Field] = [],
+  target? : String = "",
+) -> Unit {
+```
+
+#### input
+
+- `self : LibraryLogger[S]` - Library-facing logger that should emit the record.
+- `level : Level` - Severity level for the record.
+- `message : String` - Log message text.
+- `fields : Array[Field]` - Optional structured fields attached to the record.
+- `target : String` - Optional per-call target override.
+
+#### output
+
+- `Unit` - No return value. The record is either skipped by level gating or written to the wrapped sink pipeline.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This method delegates directly to the wrapped logger's `log(...)` behavior.
+- The logger checks `is_enabled(level)` before building and writing the record.
+- If `target` is empty, the facade's stored default target is used.
+- The narrower library facade does not change record construction semantics; it only limits the visible API surface.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Full Per-call Control Through The Library Facade
+
+When library code should choose level, fields, and target explicitly:
+```moonbit
+logger.log(
+  Level::Info,
+  "worker started",
+  fields=[field("job", "sync")],
+  target="service.worker",
+)
+```
+
+In this example, the call site controls every major record property while keeping the narrower facade type.
+
+#### When Build Higher-level Library Wrappers
+
+When package code defines its own logging helpers:
+```moonbit
+fn log_retry(logger : LibraryLogger[ConsoleSink], attempt : Int) -> Unit {
+  logger.log(Level::Warn, "retry scheduled", fields=[field("attempt", attempt.to_string())])
+}
+```
+
+In this example, `log(...)` acts as the shared primitive under custom library-facing wrappers.
+
+### Error Case
+
+e.g.:
+- If `level` is below the current minimum threshold, the method returns without writing a record.
+
+- If `target` is empty and the logger default target is also empty, the emitted record simply has no target label.
+
+### Notes
+
+1. Use this API when the call site needs full control instead of a fixed-severity helper.
+
+2. The facade remains synchronous; any buffering behavior only comes from the wrapped sink type.