In 15.05.01 to account for traffic I used as a sensor softflowd. As a collector, I used flow-tools from 14.07, because In later versions it is no longer there.
It worked.
In snapshot and LEDE flow-tools does not start.
I did not find any more flow collectors in standard packages.
Was I looking badly?
If you do not, help to compile.
There was a package repo cleanup before 15.05 release. Since then all packages need a "maintainer" who takes care of the package version upgrades etc. All active packages needed to be explicitly brought into the new packages feed repo in Github. https://github.com/openwrt/packages
But nobody has been interested in flow-tools, so it has been left to rot in the old abandoned "oldpackages" repository.
If you want that package for LEDE (or Openwrt DD master), volunteer to be the maintainer, edit/revise the old Makefile and patches to compile the newest version of flow-tools with the current LEDE & Openwrt and run-test the compiled package. After verifying that it works ok, create a pull request in Github packages feed to get the package back to the official repo.
flow-tools package in "oldpackages": http://git.openwrt.org/?p=packages.git;a=tree;f=net/flow-tools;h=4b590c09c852296e87c9a7861fbd4c6a1868998d;hb=HEAD
As nobody has touched the package for some 3-4 years, is it quite possible that it does not compile with the current buildroot without some modifications.