Historically, non-decimal monetary systems were common. E.g. pound / shilling.
Every currency I can think of or have ever used is 'decimal' (where a higher/lower unit of currency is either a multiple or factor of 10).
Are all currencies used by countries nowadays 'decimal'?
The word you're looking for is not "metric" but "decimal".
Pretty much all currencies are decimal these days except for currencies that have divisions where the sub-unit is no longer used as it's worth so little.
The only countries left with non-decimal currencies are Mauritania and Madagascar according to wikipedia
The Wikipedia article Non-decimal currency explains the situation in full:
Today, only two countries have non-decimal currencies: Mauritania, where 1 ouguiya = 5 khoums, and Madagascar, where 1 ariary = 5 iraimbilanja. However, these are only theoretically non-decimal, as in both cases the value of the main unit is so low that the sub-units are too small to be of any practical use and coins of the sub-units are no longer used.
The official currency of the Sovereign Military Order of Malta, which retains its claims of sovereignty under international law and has been granted permanent observer status at the United Nations, is the Maltese scudo, which is subdivided into 12 tarì, each of 20 grani with 6 piccioli to the grano.
All other contemporary currencies are either decimal or have no sub-units at all, either because they had been abolished or because they have lost all practical value and not used.
What about Japan? AFAIK the yen is not subdivided into lesser units, nor is it a subdivision of a larger unit. Even if denominations are issued in multiples of 10**n yen, that is arbitrary and doesn't fundamentally make the yen a decimal currency.
