Rhythmbox and metadata

Unfortunately no, Rhythmbox doesn't support fetching of metadata not even through plugins. But there are better alternatives available in Ubuntu repositories. I would suggest EasyTAG

EasyTAG is an utility for viewing and editing tags for MP3, MP2, FLAC, Ogg Vorbis, Speex, MP4/AAC, MusePack, Monkey's Audio and WavPack files. Its simple and nice GTK2 interface makes tagging easier under GNU/Linux or Windows.

You can install it by running the following command in terminal.

sudo apt-get install easytag

or search for "easytag" in Software Center and install in from there.

[update]

23.Dec 2017 (rhythmbox 3.4.1):

Rhythmbox is actually able to fetch metadata from online databases and write the metadata into (at least) mp3/ogg files and read metadata from mp3/ogg files.

It seems like it is not capable of reading/writing metadata from/to flac.