net: switch from log to tracing (#1455)

* net: switch from `log` to `tracing`.

Motivation:

The `tracing` crate implements scoped, structured, context-aware
diagnostics, which can add significant debugging value over unstructured
log messages. `tracing` is part of the Tokio project. As part of the
`tokio` 0.2 changes, I thought it would be good to move over from `log`
to `tracing` in the tokio runtime.

Solution:

This branch replaces the use of `log` in `tokio-net` with
`tracing`. I've tried to leave all the instrumentation points more or
less the same, but modified to use structured fields instead of string
interpolation.

Notes:

I removed the timing in `Reactor::poll` in favor of simply adding a
`#[tracing::instrument]` attribute. Since the generated `tracing` span
will have enter and exit events, a `tracing::Subscriber`
implemementation can use those to record timestamps, and process that
timing data in a much more sophisticated manner than including it in a
log line.

We can add the timestamps back if they're desired.

Signed-off-by: Eliza Weisman <[email protected]>

tokio-net/src/process/unix/orphan.rs

@@ -1,5 +1,4 @@
 use crossbeam_queue::SegQueue;
-use log::error;
 use std::io;
 use std::process::ExitStatus;
 
@@ -71,9 +70,9 @@ impl<T: Wait> OrphanQueue<T> for AtomicOrphanQueue<T> {
             match orphan.try_wait() {
                 Ok(Some(_)) => {}
                 Err(e) => error!(
-                    "leaking orphaned process {} due to try_wait() error: {}",
-                    orphan.id(),
-                    e,
+                    message = "leaking orphaned process due to try_wait() error",
+                    orphan.id =orphan.id(),
+                    error = %e,
                 ),
 
                 // Still not done yet, we need to put it back in the queue