Function format_micros 

fn format_micros<F>(buf: &mut F, micros: u32)
where
    F: FormatBuffer,

Writes a microsecond fraction to buf with trailing zeros stripped, or nothing at all when it is zero.

micros must be below one second, which is what split_nanos_to_micros guarantees. A larger value would be written as a fraction it does not fit in, silently shifting the rendered time by about a second.