The inet_diag module looks like it first arrived in the 3.11 kernel.
The random sampling function in BPF looks like it was first
implemented in 3.18.
The tcp_info lookup is very efficient, you don't need to walk the
table to find a record, you can ask for a specific record by filling
in a tcp socket structure. The randomly sampled packet headers provide
the information needed to make the request.
You might also be interested to know that your recommendations for
scheduling counter export have been implemented by the Host sFlow and
Open vSwitch, e.g.
Counter export from fast changing Docker container pools also posed an