add docs for filter.expr and wrapped payload stuff

src/content/docs/processors/filter/expr.md

@@ -0,0 +1,18 @@
+---
+title: Expr Filter
+sidebar:
+    order: 2
+---
+The `filter.expr` processor evaluates an [Expr expression](https://expr-lang.org/playground). If the expression evaluates to true then the payload is sent through. This processor will return an error if the expression fails to evaluate.
+
+- **type**: `filter.expr`
+- **params**:
+    - **expression**: [Expr expression](https://expr-lang.org/playground) to evaluate. The expression has access to the [wrapped payload](/concepts/payload).
+
+### Example
+Match a payload great than or equal to 0
+```
+- type: filter.expr
+  params:
+    pattern: "Payload >= 0"
+```