Use weak_ptr to cache the file handle of file segment. The max cached number of file handles can be configured by `file_cache_max_file_reader_cache_size`, default `1000000`. Users can inspect the number of cached file handles by request BE metrics: `http://be_host:be_webserver_port/metrics`: ``` # TYPE doris_be_file_cache_segment_reader_cache_size gauge doris_be_file_cache_segment_reader_cache_size{path="/mnt/datadisk1/gaoxin/file_cache"} 2500 ```