fix(metadata): add metadata validator.

unblob/models.py

@@ -33,6 +33,13 @@ def metadata_converter(obj: Union[Dict, Instance]) -> dict:
     raise ValueError("Can only convert dict or Instance")
 
 
+def metadata_validator(instance, attribute, value):
+    if attribute.name == "metadata" and isinstance(instance, Chunk):
+        for k, _ in value.items():
+            if not isinstance(k, str):
+                raise TypeError("metadata keys must be string")
+
+
 @attr.define(frozen=True)
 class Task:
     path: Path
@@ -100,7 +107,9 @@ class ValidChunk(Chunk):
 
     handler: "Handler" = attr.ib(init=False, eq=False)
     is_encrypted: bool = attr.ib(default=False)
-    metadata: dict = attr.ib(factory=dict, converter=metadata_converter)
+    metadata: dict = attr.ib(
+        factory=dict, converter=metadata_converter, validator=metadata_validator
+    )
 
     def extract(self, inpath: Path, outdir: Path):
         if self.is_encrypted: