You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: lib.rs
    path of file: ./repos/linkerd2-proxy/linkerd/stack/tracing/src
    
    the code of the file until where you have to start completion: #![deny(rust_2018_idioms, clippy::disallowed_methods, clippy::disallowed_types)]
#![forbid(unsafe_code)]

use linkerd_stack::{layer, NewService, Proxy};
use std::task::{Context, Poll};
use tracing::instrument::{Instrument as _, Instrumented};
use tracing::{trace, Span};

/// A strategy for creating spans based on a service's target.
pub trait GetSpan<T> {

    
    fn call(&mut self, request: Req) -> Self::Future {
        let span = self
            .current_span
            .take()
            // NOTE(eliza): if `current_span` is `None` here, we were called
            // before  being driven to readiness, which is invalid --- we're
            // permitted to panic here, so we could unwrap this. But, it's not
            // important...we can just make a new span, so I thought it was
            // better to err on the side of not panicking.
            .unwrap_or_else(|| self.get_span())
            .entered();

        trace!(?request, "service");
        self.inner.call(request).instrument(span.exit())
    }