The chemical reactions and pathways involving extensins, a group of 60-90 kDNA hydroxyproline (Hyp)-rich glycoproteins whose polypeptide backbone consists of many repeats of structural Ser(Hyp)4-6 motifs, with heavily glycosylated 1-4 arabinose residues O-linked to contiguous stretches of Hyp residues, with most of the Ser residues being O-galactosylated.