📝 Add filter predicate API docs

docs/api/has-field.md

@@ -0,0 +1,79 @@
+---
+name: has-field
+group: api
+category: filtering
+update-time: 20260512
+description: Create a reusable record predicate that checks whether a field key exists.
+key-word:
+    - field
+    - filter
+    - predicate
+    - public
+---
+
+## Has-field
+
+Create a `RecordPredicate` that returns `true` when a record contains at least one field with the given key. Use it when presence alone matters more than a specific field value.
+
+### Interface
+
+```moonbit
+pub fn has_field(key : String) -> RecordPredicate {}
+```
+
+#### input
+
+- `key : String` - Field key that should exist in the record.
+
+#### output
+
+- `RecordPredicate` - Predicate that matches records containing a field whose key equals `key`.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The predicate scans `rec.fields` in order and stops on the first matching key.
+- Only the key is checked; the field value is ignored.
+- This helper works well for optional metadata such as `request_id`, `tenant`, or `trace_id`.
+- It can be combined with `field_equals(...)` when some records need stricter field matching.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Keep Records Carrying Correlation Data
+
+When only records with tracing context should pass:
+```moonbit
+let logger = Logger::new(console_sink())
+  .with_filter(has_field("trace_id"))
+```
+
+In this example, records without `trace_id` are filtered out.
+
+#### When Combine With Severity Rules
+
+When contextual records should also meet a level threshold:
+```moonbit
+let predicate = all_of([
+  has_field("request_id"),
+  level_at_least(Level::Warn),
+])
+```
+
+In this example, only warning-or-higher records with request context remain.
+
+### Error Case
+
+e.g.:
+- If `key` is empty, the predicate only matches fields whose key is also empty.
+
+- If the same key appears multiple times, the predicate still returns `true` after the first match.
+
+### Notes
+
+1. Use this helper when field presence is meaningful even if the actual value changes every time.
+
+2. Field ordering does not change the result, only the early return cost.
+