Add naming conventions for flags on spans (#13362)

* Add convention for flags

* Formatting

Author: cmanallen

develop-docs/sdk/telemetry/traces/span-data-conventions.mdx

@@ -102,6 +102,13 @@ Below describes the conventions for the Span interface for the `data` field on t
 | `thread.id`       | string  | Identifier of a thread from where the span originated.             | `123456` |
 | `thread.name`     | string  | Label identifying a thread from where the span originated.         | `main`   |
 
+## Flags
+
+| Attribute                           | Type    | Description                                     | Examples     |
+| ------------------------------------| ------- | ------------------------------------------------| ------------ |
+| `flag.evaluation.<flag-name>`       | string  | Identifier of a flag evaluated within the span. | `can_access` |
+
+
 ### Deprecated
 
 Names that SDKs are still sending so we cannot remove them yet, but should not be used in new code.