ovn-controller installs OpenFlow rules (flows and groups) in ovs through the means of a flow bundle. The usual sequence of operations is.

1. create flow bundle
2. populate needed OpenFlow modification (flows, groups, meters) commands and add them to the bundle
3. commit flow bundle
4. encode an OVS OF barrier request (OFPRAW_OFPT11_BARRIER_REQUEST)
5. send all openflow messages generated by steps 1-4

ovn-controller should be able to provide information about how long it took from the moment when the OF updates were generated and until OVS processed all the updates.

Probably a good approximation of that is to measure the time between the moment the barrier was generated (step #4) and the moment ovn-controller receives the corresponding barrier-reply (OFPT_BARRIER_REPLY) from OVS.

One potential way of reporting this data is through a new "ovn-appctl command", e.g., ovn-appctl stats/flow-install-time. This could report the last maximum measured since the previous run.

NOTE: ovn-controller should not include the time it takes to run its own processing (e.g., engine_run()) into the time it reports for installing flows.