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1. Field of the Invention This invention relates to liquid metering devices for use in metering the flow of viscous fluids and is more particularly concerned with devices for metering the flow of ink being fed under pressure from a storage vessel to a printing press. 2. Description of the Prior Art Fluid flow meters are well-known in the art and have been widely used to measure flow of liquids such as water, oil, fuels, hydraulic fluids and the like. Such meters include glass tube variable area meters, turbine meters, ultrasonic meters, coriolis effect meters and the like. However, such meters are generally unsatisfactory for use in metering the flow of relatively viscous materials such as printing inks which generally have viscosities in the range of about 100 to 200 poise. Main U.S. Pat. 4,042,149 describes a liquid pump and metering system for use with hot asphalt, a relatively viscous material which is difficult to handle. The system employs a first positive displacement pump which is responsible for pumping fluid within the system and a second positive displacement pump through which the fluid is pumped in a direction which is the reverse of that in which the second pump would normally operate. The second pump acts as a metering device the rate of rotation of the rotatable portion of the pump being a measure of liquid flow therethrough. The rate of rotation in question is sensed by a device mounted on the exterior of the pump shaft and signals generated by the device are processed by electronic means to determine rate of fluid flow. Krygeris U.S. Pat. 3,835,777 discloses a method of controlling ink supply to a printing press by employing sensors to monitor the density of ink being deposited on the material being printed. Signals generated by the sensors are compared with signals representing upper and lower limits of range of the desired density and control signals are derived to correct any deviation from the desired range. Rodvelt U.S. Pat. 4,524,692 teaches controlling the amount of ink transferred to the ink train in a printing press. The system includes a fountain roller in contact with the ink supply and a ductor roller which can be moved between the ink train and fountain roller. The control mechanism involves sensing the interval of rotational movement of the fountain roller when in contact with the ductor roller as a measure of the amount of ink transferred to the ink train. Fluid metering systems not specifically directed to use with viscous fluids such as ink but employing various types of sensing devices for detecting flow rates are disclosed in U.S. Pats. 4,520,677; 3,780,579; 3,906,198; 4,120,032; and 4,413,264. It has now been found that the amount of relatively viscous fluids being supplied to a system, such as ink passing through feed lines such as those connecting a source of ink to a printing press, can be determined accurately and conveniently using a method and apparatus which will be described hereinbelow.