chore: type hint

fgpyo/util/metric.py

@@ -218,7 +218,7 @@ def read(
         with io.to_reader(path) as reader:
             try:
                 header = cls.read_header(reader, comment_prefix=comment_prefix)
-                fieldnames = header.fieldnames
+                fieldnames: List[str] = header.fieldnames
             except ValueError:
                 raise ValueError(f"No header found in file: {path}")