This is useful for traversing all the meta data elements in a value list, since there is no element to look up the metadata object.
For example usage, see https://github.com/Stackdriver/collectd/pull/166
int uc_meta_data_delete(const value_list_t *vl,
const char *key) UC_WRAP(meta_data_delete)
+
+/* The second argument is called `toc` in the API, but the macro expects
+ * `key`. */
+int uc_meta_data_toc(const value_list_t *vl, char ***key) UC_WRAP(meta_data_toc)
+
#undef UC_WRAP
/* We need a new version of this macro because the following functions take