The ornamental design for a flowmeter, as shown and described.
FIG. 1. is a left side perspective view of the flowmeter of our new design;
FIG. 2. is a left side elevation view thereof, the right side elevation view being the same;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken line showing of components of a flowmeter are included for the purpose of illustrating non-claimed subject matter and forms no part of the claimed design.