The choice is yours. However, keoxkeox, the rule about so-called countable and uncountable was never intended to cover all the nouns there are. It covers a difference between such stuff as water, air or grass, which can only be measured by mass or , etc., (so such nouns are often called mass nouns) and things, like pebbles, nuts and houses that can be counted (so such nouns are often referred to as count nouns). With interest, the issue of counting versus measuring does not arise,
Even where it does, this distinction is beginning to break down. “I’ll have two cokes, three lagers and two waters, please” can regularly be heard in many a bar. The customer is not unconsciously thinking ‘glasses of’, or ‘bottles of’.
So ‘showing an interest’ and ‘showing interest’ are equally familiar expressions.
I do not know if this is a relevant consideration. But There is a difference between English and other European languages: The indefinite article is not the number 1. In French, German, Italian and Greek it is. Mind you, they all say un café, ein café, uno espresso, or ένα καφεδάκι . So perhaps my point is not relevant.