📝 Add filter predicate API docs

docs/api/field-equals.md

@@ -0,0 +1,80 @@
+---
+name: field-equals
+group: api
+category: filtering
+update-time: 20260512
+description: Create a reusable record predicate that matches a field by exact key and value.
+key-word:
+    - field
+    - filter
+    - predicate
+    - public
+---
+
+## Field-equals
+
+Create a `RecordPredicate` that returns `true` when a record contains a field whose key and value both match exactly. Use it for stable attribute-based routing.
+
+### Interface
+
+```moonbit
+pub fn field_equals(key : String, value : String) -> RecordPredicate {}
+```
+
+#### input
+
+- `key : String` - Field key to inspect.
+- `value : String` - Exact value expected for that key.
+
+#### output
+
+- `RecordPredicate` - Predicate that matches records containing a field with the expected key and value.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Matching requires both `field.key == key` and `field.value == value`.
+- The predicate returns `true` on the first matching field.
+- This helper is useful for routing records by environment, tenant, operation name, or fixed tags.
+- It is stricter than `has_field(...)` because presence alone is not enough.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Keep One Tenant Stream
+
+When log routing should isolate one tenant:
+```moonbit
+let logger = Logger::new(console_sink())
+  .with_filter(field_equals("tenant", "acme"))
+```
+
+In this example, records for other tenants are excluded.
+
+#### When Combine With Exact Target Matching
+
+When field and target must both match:
+```moonbit
+let predicate = all_of([
+  target_is("billing"),
+  field_equals("region", "cn"),
+])
+```
+
+In this example, only billing records tagged for the `cn` region remain.
+
+### Error Case
+
+e.g.:
+- If `key` or `value` is empty, matching still uses exact equality and may produce no results unless records contain the same empty string.
+
+- If a record contains the key with multiple values, any one exact match is enough for the predicate to return `true`.
+
+### Notes
+
+1. Prefer exact field matching over message substring matching for long-term routing rules.
+
+2. Keep field naming stable across producers if this predicate is reused in shared configs.
+