Improve performance and reduce allocations

[davidpdrsn]

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

Hello 👋 These changes improve performance and reduce allocations quite a bit. Mostly by writing directly to the underlying writers rather than intermediate strings. The code is slightly less clean but maintains most of the previous structure.

The numbers

Baseline using version from crates.io:

time=601.537292ms
allocations=4.700.014

After these changes:

time=283.841666ms
allocations=70.0014

Benchmark script I used:

use std::time::Instant;

use alloc_counter::AllocCounterSystem;
use tracing::*;
use tracing_logfmt::{EventsFormatter, FieldsFormatter};

#[global_allocator]
static A: AllocCounterSystem = AllocCounterSystem;

fn main() {
    let subscriber = tracing_subscriber::fmt::Subscriber::builder()
        .event_format(EventsFormatter)
        .fmt_fields(FieldsFormatter)
        .with_writer(std::io::sink)
        .finish();

    tracing::subscriber::with_default(subscriber, || {
        let start = Instant::now();

        let ((allocations, _, _), _) = alloc_counter::count_alloc(|| {
            for _ in 0..100_000 {
                info!("message");
                {
                    let _span = info_span!("one", a = 1, b = 2).entered();
                    info!("message in span");
                    {
                        let _span = info_span!("two", a = 1, b = 2).entered();
                        info!("message in span");
                    }
                }
                debug!(a = 1, b = 2, c = false, "message with fields");
            }
        });

        println!("time={:?}", start.elapsed());
        println!("allocations={allocations}");
    });
}

[fredr]

Awesome work ❤️

Just had one nit/question that we can discuss

[fredr]

🎉