read_metrics_one_file_by_end_time do not work as expected because read_line function reads \n into the buffer

[flearc]

From official doc, the read_line function read \n to the buffer as well.

use std::io::{self, BufRead};

let mut cursor = io::Cursor::new(b"foo\nbar");
let mut buf = String::new();

// cursor is at 'f'
let num_bytes = cursor.read_line(&mut buf)
    .expect("reading from cursor won't fail");
assert_eq!(num_bytes, 4);
assert_eq!(buf, "foo\n");
buf.clear();

\n will lead to failures in functions like read_metrics_one_file_by_end_time.

Consider use the lines function in BufReader.