Add docs bmv2 direct counters meters (#744)
Add docs bmv2 direct counters meters (#744)

* Add documentation for direct counters and meters

... that are probably specific to the simple_switch implementation.

* Use the code phrase "BMv2 v1model implementation"

p4lang/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model