probe::json_data — Fires whenever JSON data is wanted by a reader.
json_data
This probe fires when the JSON data is about to be read. This
probe must gather up data and then call the following macros to
output the data in JSON format. First, @json_output_data_start
must be called. That call is followed by one or more of the
following (one call for each data item):
@json_output_string_value, @json_output_numeric_value,
@json_output_array_string_value, and
@json_output_array_numeric_value. Finally @json_output_data_end
must be called.