At present (180818), it is is only possible to measure either 1\. the length of a single track segment or\, 2\. the sum of the lengths of all track segments in a multi segment track\, irrespective of whether the overall track is a single continuous trace or is a set of branching segments connecting multiple nodes (pads or vias). It is not possible to measure the length of a set of contiguous track segments between an arbitrary pair of nodes. The series of screenshots below illustrates the problem. Click on a single net segment:  The length of that single segment is shown. Click on a net segment and then press "H" to highlight the whole track:  The whole net is highlighted and what looks like the total length of all segments is shown. Click on a net segment and then CTRL+click on another contiguous segment to highlight a branch between two nodes:  No length is shown. Now go to the Design Manager and click on the net:  The whole net is highlighted and what looks like the total length of all segments is shown again.