obs: improve spans use in flashbots submit

[dylanlott]

This PR improves span usage in the Flashbots submit task for better tracking in groundcover and analytics.

[dylanlott]

• obs: improve spans use in flashbots submit #165 👈 (View in Graphite)
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[Evalir]

this could be further simplified with:

let bundle = match self.prepare(&sim_result)
    .instrument(span.clone())
    .await
    .inspect_err(|error| {
        counter!("signet.builder.flashbots.bundle_prep_failures").increment(1);
        span_debug!(span, %error, "bundle preparation failed");
    }) {
    Ok(bundle) => bundle,
    Err(_) => continue,
};

[prestwich]

whenever you're happy :)