Artist.StatsSourceOperations on artist's stats
type of artist's stats
val init :
music_count:int ->
album_count:int ->
duration:int ->
play_count:int ->
size:int64 ->
avg_rating:float ->
played_at:float option ->
tinit ~music_count ~duration ~play_count ~size ~avg_rating ~played_at creates a new album stats.
album_count stats returns the number of albums where artist is an album-artist.