METERING DEVICE FOR FEEDING LOOSE GRANULAR SUBSTANCES TECHNICAL FIELD The present invention relates to the engineering of vertical packaging machines and specifically refers to metering devices for feeding loose granular substances. International reference classification B 65 b. BACKGROUND ART Different types of metering devices are presently used to predetermine the exact quantity of free-flowing granular products to be introduced into a package: metering screw or bucket feeders or checkweighers. All of these types of metering devices share a drawback in that they release the dosed product at the upper end of the forming tube of the vertical packaging machine, thus far from the bottom of the bag to be filled, and this inevitably entails all of the problems caused by the product having to travel through an air-filled forming tube. The solution proposed by the present invention eliminates all of the aforesaid problems. DESCRIPTION The invention will now be described in detail referring to the diagrams and appended drawings, intended as non-restrictive examples. Figure 1 provides a schematic cross-section view of a vertical packaging machine equipped with a metering device for feeding loose granular substances. One may expressly note the absence of traditional metering screw or bucket feeders or standard weighers that feed the quantity of product to be packaged. A circumstance to be highlighted is that figure 1 represents the position that the metering device of the present invention assumes when the diaphragm (5) cuts off the flow of product (1) gravity fed through the shaped channel (2). Also worthy of particular consideration is the fact that the bottom of the package to be gravity filled is at the same level as the transverse sealing clamps (12) and the product drop height (h) is reduced to the distance between the clamps and the lower extremity (9) of the product feed channel (1), which corresponds to the end of the forming tube (3). This minimal drop of the loose granular product is a highly welcome feature that optimises the performance of the metering device of the present invention, especially for loose granular products, even if they contain powder. Figure 2 shows the initial phase of the operation whereby the product (1) is introduced and descends freely by gravity through the channel (2). It may be noted that the drop height is indicated by the letter (h) and corresponds to the vertical distance between the cut-off diaphragm (5) that slides beneath the lower extremity (9) of the feed channel (2) and the transverse sealing clamps (12). It should be noted that the discharge of the loose granular product (1) is induced by gravity but permitted only by the opening of the cut-off diaphragm (5) regulated by the motor (M) which controls the timed rotation of the shaft (7). Figure 3 shows the continuation of the product's gravity-induced free fall. Figure 4 illustrates the end of the metering operation. It may be noted that the diaphragm (5) is closed and has cut off the gravity-induced free flow of loose product (1). Figure 5 illustrates the operation whereby the package (6) filled with a predetermined quantity of product is then sealed. Figure 6 schematically illustrates the use of the metering device to introduce a predetermined quantity of loose granular product into a rigid container (61). It may be noted that the letter (h) indicates that the product drop height is always minimal and is defined precisely by the vertical distance between the bottom of the container (6') and the lower extremity (9) of the feed channel (2) sealed off by the diaphragm (5). Figure 7 provides a more detailed illustration of a vertical cross-section of the diaphragm (5) which cuts off the flow of loose product (1). It may be noted that the upper surface of said revolving diaphragm slides over the lower extremity (9) of the feed channel (2) so as to seal it off. It is also evident that the rotation of the revolving diaphragm (5) is driven by the shaft (7) controlled by a timed motor. Figure 8 shows the diaphragm (5) as seen from below. In the diaphragm (5) one may note the presence of an aperture (8) whose shape matches that of the feed channel (2). When this aperture is positioned directly beneath the feed channel (2) the product can flow freely, drawn by natural gravity; when the diaphragm (5) slides closed to seal off the lower extremity (9) of the shaped channel (2) the free flow of product is cut off. The opening time is regulated by the motor (M), which predetermines the exact quantity of product to be introduced into the resulting package (6). The device of the present invention thus functions like a traditional gravity metering device but is inserted inside the forming tube of the vertical packaging machine so that the product is discharged at the end of said forming tube. This arrangement minimises the product drop height to the value indicated by (h) in the previous figure (7). Figure 9 shows a cross-section view of the metering device. One may note the presence of a tube (10) for introducing gas and an open zone (11) for venting and circulating air during product discharge operations (1). It is also possible to note the necessary geometrical congruence between the shape of the aperture (8) and the shape of the feed channel (2). It is moreover evident that the shaft (7) is integral with the revolving diaphragm (5) precisely in order to drive the latter and regulate the duration of its opening and closing, which will define the dose of the fed product. In the figures each individual item is marked as follows: 1 indicates a loose granular product which may also contain powder. 2 indicate a specially shaped feed channel through which the product to be dosed flows down freely. 3 indicates the forming tube of a vertical packaging machine. 4 schematically represents the structure of a vertical packaging machine. 5 indicates a diaphragm for cutting off the flow of fed product. 6 indicates a flexible packaging container. ' indicates a rigid container. 7 indicates the shaft driving the rotation of the cut-off diaphragm (5). . indicates the aperture that allows product to flow through when the diaphragm (5) is open. indicates the bottom surface of the feed channel (2). 10 indicates a tube for injecting gas into the package. 11 indicates the air venting and circulation zone inside the forming tube (3). 12 indicates the clamps for transverse sealing of the packages, h indicates the drop height of the loose granular product. M indicates a timed motor. The invention naturally lends itself to different embodiments as regards both the dimensions and structural proportions, as well as the technological choices in respect of the materials to be used in the manufacture of the various constituent parts and various commercial components. It shall be emphasized that the innovative concept of the present invention is founded upon minimising the drop height (h), which is achieved through a technological contrivance whereby the flow of the loose product is cut off at the bottom of the feed channel (2). Moreover, said cut-off of flow is obtained by means of a revolving diaphragm (5) that slides over the lower terminal surface (9) of the aforesaid feed channel (2) to seal it off. Thus the scope of protection of this patent shall extend to all metering devices for feeding loose granular substances that display the inventive characteristics of the present invention as substantially described, illustrated and claimed below.