Compatibility with tokio-tracing

[andylokandy]

tracing is a widely used tracing library in the Rust community. Many fastrace users have requested compatibility with existing code and libraries that use tokio-tracing (#13). This issue aims to address these needs.

Compatibility Goals:

1. Mixed Usage: Support applications using fastrace alongside libraries that use tokio-tracing.
2. Seamless Migration: Enable applications using tokio-tracing to migrate to fastrace with minimal changes.
3. Library Support: Allow library authors to support both fastrace and tokio-tracing without duplicating code.

Required Features:

• Span and Log Collection: Allow fastrace to collect spans and logs from tokio-tracing.
• API Compatibility Layer: Implement a compatibility layer in fastrace to mimic the API and macros of tokio-tracing.
• Span Reporting: Enable fastrace to report spans to tokio-tracing.

cc @Xuanwo @tisonkun @zhongzc