"flat_key","key","int_value","string_value"
"chrome_histogram_sample.name","chrome_histogram_sample.name","[NULL]","histogram_name1"
"chrome_histogram_sample.name","chrome_histogram_sample.name","[NULL]","histogram_name2"
"chrome_histogram_sample.name_hash","chrome_histogram_sample.name_hash",10,"[NULL]"
"chrome_histogram_sample.name_hash","chrome_histogram_sample.name_hash",20,"[NULL]"
"chrome_histogram_sample.name_hash","chrome_histogram_sample.name_hash",30,"[NULL]"
"chrome_histogram_sample.sample","chrome_histogram_sample.sample",100,"[NULL]"
"event.category","event.category","[NULL]","cat1"
"event.category","event.category","[NULL]","cat2"
"event.category","event.category","[NULL]","cat3"
"event.name","event.name","[NULL]","slice1"
"event.name","event.name","[NULL]","slice2"
"event.name","event.name","[NULL]","slice3"
"is_root_in_scope","is_root_in_scope",1,"[NULL]"
"source","source","[NULL]","descriptor"
"trace_id","trace_id",12345,"[NULL]"
"utid","utid",1,"[NULL]"