ASPIRATIONFLOW SENSOR TECHNICAL FIELD BACKGROUND SUMMARY [0001] This application claims priority to U.S. Application No. 16/381,809, entitled, “ASPIRATIONFLOW SENSOR,”and filed April 11, 2019. [0002] This disclosure relates to medical aspiration. [0003] A medical catheter defining at least one lumen has been proposed for use with various medical procedures. For example, in some cases, a medical catheter may be used to access and treat defects in blood vessels, such as, but not limited to, lesions or occlusions in blood vessels. This treatment may involve drawing fluid through the medical catheter. [0004] In some aspects, this disclosure describes example fluid flow sensors and systems that include a catheter and a fluid flow sensor. The fluid flow sensor includes a fluid inlet configured to receive fluid from the catheter and a fluid outlet configured to discharge the fluid. The fluid flow sensor also includes a flow oscillator configured to oscillate flow of the fluid through the fluid flow sensor to generate flow-induced vibrations. This disclosure also describes examples of methods of using the fluid flow sensors and systems including the fluid flow sensors. [0005] Clause 1: In one example, an aspiration system comprises: a catheter; and a fluid flow sensor comprising: a fluid inlet configured to receive fluid from the catheter; a fluid outlet configured to discharge the fluid; and a flow oscillator configured to oscillate flow of the fluid through the fluid flow sensor to generate flow-induced vibrations. [0006] Clause 2: In some examples of the aspiration of clause 1, the flow oscillator further comprises: a flow diverting portion configured to oscillate the flow of the fluid between the fluid inlet and the fluid outlet to generate an oscillating pressure differential; and an oscillating portion downstream of the flow diverting portion and configured to oscillate in response to the oscillating pressure differential. [0007] Clause 3: In some examples of the aspiration system of clause 2, the oscillating portion comprises at least one of a flap, a roller, a ball, or a paddle.