The isomers which have been cited as examples of metamers in chemical literature consist primarily of ethers; but this could by the same reasoning be extended to thioethers, secondary as well as tertiary amines, esters, secondary as well as tertiary amides, (mixed) acid anhydrides etc.
Ketones however, should be excluded from this class of isomeric relationship, as they primarily are part of position isomerism - as there is no heteroatom present in the functional group, so the two alkyl groups (main chain and side chain) are not disconnected from each other.