[Thinkit] Add sFlow gNMI config in sFlow test. Increase sFlow tolerance rate. Add a test to verify sampling size works. Fix sample size issue. Adds sFlow gNMI config constants.

[Thinkit] Add sFlow gNMI config in sFlow test. Increase sFlow tolerance rate. Add a test to verify sampling size works. Fix sample size issue. Adds sFlow gNMI config constants. #3913