Add the content_length field to the Document base class

earthsnake/document/es4.py

@@ -221,6 +221,10 @@ class Es4Document(Document):  # pylint: disable=too-many-instance-attributes
         self._content = value
         self._content_hash = self.hash_value(value)
 
+    @property
+    def content_length(self) -> int:
+        return len(self._content)
+
     def generate_hash(self) -> str:
         """Calculate the hash of the document