fix: use compatible tracing version

[stepantubanov]

Seems like tracing changed public interface in tokio-rs/tracing#2212

And poem is using the new interface already, so it fails with tracing <= 0.1.35.

    Checking poem v1.3.55 (https://github.com/poem-web/poem.git?branch=master#9941b489)
error[E0277]: the size for values of type `str` cannot be known at compilation time
    --> /Users/stepantubanov/.cargo/git/checkouts/poem-5d35782906f249bc/9941b48/poem/src/middleware/tracing_mw.rs:50:41
     |
50   |             span.record("path_pattern", path_pattern.0.as_ref());
     |                  ------                 ^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
     |                  |
     |                  required by a bound introduced by this call
     |
     = help: the trait `Sized` is not implemented for `str`
note: required by a bound in `Span::record`
    --> /Users/stepantubanov/.cargo/registry/src/github.com-1ecc6299db9ec823/tracing-0.1.35/src/span.rs:1194:30
     |
1194 |     pub fn record<Q: ?Sized, V>(&self, field: &Q, value: &V) -> &Self
     |                              ^ required by this bound in `Span::record`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `poem` due to previous error

[attila-lin]

Thanks！