。
Before I forget, let me just mention...
Literal
Not-forget [while-うちに] say-in-advance-[polite-ます], [suspensive-と]
Built around the set phrase 忘れないうちに ('while I'm not yet forgetting' = before I forget). うちに attached to a negative verb expresses 'before X becomes true'. ~ておく marks the speech act as preparatory — saying it now to lock it in. The trailing と at the very end is suspensive: it signals that more is coming, like English 'so...' trailing into the next thought. Subject and what's about to be said are both omitted — context supplies both.