1. solved the previous delayed unit test file size is too large (1.7G+) and the unit test link time is too long problem problems
2. Unify all unit tests into one file to significantly reduce unit test execution time to less than 3 mins
3. temporarily disable stream_load_test.cpp, metrics_action_test.cpp, load_channel_mgr_test.cpp because it will re-implement part of the code and affect other tests
#5146
Add histogram metrics into util/metrics.h. The data structure of histogram is implemented in util/histogram.h,
which could also be used in other situations that in need of histogram. Unit tests added as well.
