/** Add two combined counters, results in the first counter
@param [in,out] a - (vlib_counter_t *) dst counter
@param b - (vlib_counter_t *) src counter
/** Add two combined counters, results in the first counter
@param [in,out] a - (vlib_counter_t *) dst counter
@param b - (vlib_counter_t *) src counter