add encode fn to metadata (#36)

Author: camfairchild

bt_decode.pyi

@@ -306,15 +306,26 @@ class MetadataV15:
 
     @staticmethod
     def decode_from_metadata_option(encoded_metadata_v15: bytes) -> "MetadataV15":
+        """
+        Decodes to Option<Vec<u8>>, then decodes to MetadataPrefixed and returns MetadataV15.
+        """
         pass
+
     def to_json(self) -> str:
         """
         Returns a JSON representation of the metadata.
         """
         pass
+
     def value(self) -> Dict[str, Any]:
         pass
 
+    def encode_to_metadata_option(self) -> bytes:
+        """
+        MetadataV15 -> MetadataPrefixed -> encoded bytes as an Option<Vec<u8>>
+        """
+        pass
+
 class PortableRegistry:
     """
     PortableRegistry is a portable for of the chains registry that

src/lib.rs

@@ -319,6 +319,12 @@ mod bt_decode {
             serde_json::to_string(&self.metadata).unwrap()
         }
 
+        fn encode_to_metadata_option(&self) -> Vec<u8> {
+            let as_prefixed = RuntimeMetadataPrefixed::from(self.metadata.clone());
+            let as_option = Some(as_prefixed.encode());
+            as_option.encode()
+        }
+
         #[staticmethod]
         fn decode_from_metadata_option(encoded_metadata_v15: &[u8]) -> Self {
             let option_vec = Option::<Vec<u8>>::decode(&mut &encoded_metadata_v15[..])