Rename module metadata to _metadata

All the other modules are private, so there is no reason why this
shouldn't be either.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/client/microgrid/_client.py

@@ -48,8 +48,8 @@
 )
 from ._connection import Connection
 from ._constants import RECEIVER_MAX_SIZE
+from ._metadata import Location, Metadata
 from ._retry import LinearBackoff, RetryStrategy
-from .metadata import Location, Metadata
 
 DEFAULT_GRPC_CALL_TIMEOUT = 60.0
 """The default timeout for gRPC calls made by this client (in seconds)."""