Here is a list of all documented struct and union fields with links to the struct/union documentation for each field:

filter_length : frequency_count_configuration_t
frequency_update_callback : frequency_count_configuration_t
pcnt_channel : frequency_count_configuration_t
pcnt_gpio : frequency_count_configuration_t
pcnt_unit : frequency_count_configuration_t
rmt_channel : frequency_count_configuration_t
rmt_clk_div : frequency_count_configuration_t
rmt_gpio : frequency_count_configuration_t
rmt_max_blocks : frequency_count_configuration_t
sampling_period_seconds : frequency_count_configuration_t
sampling_window_seconds : frequency_count_configuration_t
window_start_callback : frequency_count_configuration_t