Who tells Polonius that Hamlet looked “pale as shirt, his knees knocking each other” when Hamlet visited the chamber meant for that person? [British Drama (2018-19) Assignment 1]

It is actually Polonius himself who says this to the audience in Act 2, Scene 2 of Shakespeare's Hamlet. Polonius describes Hamlet's behavior when he visited the chamber in order to confirm his suspicion that Hamlet is lovesick for Polonius' daughter, Ophelia. The line is: "And in his [Hamlet's] grained [stained] face you could see/ The very openings and the thought of hearts/ And on his brow shame was asham'd to sit,/ For 'tis a throne where honor may be crown'd/ Sole monarch of the universal earth."