WTF: Return back metadata_extractors and metadata_indexers

It was originally removed in 616fb1b as a part of
"remove all metadata stuff" step but really this does not need any metadata
module (so no dependencies) and IMHO well worth being in the WTF. Demand for such
information is stated in datalad/datalad-metalad#340

datalad/local/wtf.py

@@ -346,6 +346,8 @@ def describe_keyring_backend(be):
     'configuration': None,
     'location': None,
     'extensions': _describe_extensions,
+    'metadata_extractors': lambda: _describe_metadata_elements('datalad.metadata.extractors'),
+    'metadata_indexers': lambda: _describe_metadata_elements('datalad.metadata.indexers'),
     'dependencies': _describe_dependencies,
     'dataset': None,
     'credentials': _describe_credentials,