## Motivation

Currently, the `tracing` crate's "log" feature always logs span
lifecycle (creation/enter/exit/close) events, even when the span has no
fields. This can be quite noisy. Since the enter, exit, and close logs
don't include the span's fields, it doesn't add that much additional
context to the surrounding log records.

## Solution

In order to continue recording this data while improving the
signal-to-noise ratio of `tracing`-generated logs, this commit changes
the log integration to log span lifecycle events under a separate
target, "tracing::span". This way, log users can opt in to this data
when it is desired, but logs are generally much less noisy. Span
creation is still logged to the default target when the span has fields,
as this often provides useful information, but is sent to the
"tracing-span" target when there are no fields.

Fixes #607

When using `tracing` with the `log-always`, the IDs of enabled spans are
available, but not included in the span lifecycle `log` records. These
could be used to enable tracking individual span's lifecycles via these
log records, but this is not currently possible.

This commit updates the `log` integration so that when a span is enabled
by a subscriber, and `log` integration is enabled, that span's `log`
records will include its ID. Now, when multiple spans with the same name
are in the logs, we have a rudimentary ability to correlate those span's
lifecycle events, without introducing so much complexity that just using
a `tracing` subscriber would be a better solution.

For example, when a subscriber is assigning IDs to spans, the logs look
like this:

```
ERROR log_with_trace foo=5
ERROR log_with_trace foo=42
ERROR log_with_trace foo=39
WARN  log_with_trace hello world;
INFO  log_with_trace hello world; thingy=42 other_thingy=666
TRACE tracing::span ++ foo; span=1
TRACE tracing::span -> foo; span=1
TRACE log_with_trace hello san francisco; foo=3 bar=4
TRACE tracing::span <- foo; span=1
TRACE tracing::span -- foo; span=1
TRACE log_with_trace ++ foo; bar=3 baz=false; span=2
TRACE tracing::span -- foo; span=2
TRACE log_with_trace hello world foo=1 bar=2
```

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Co-Authored-By: David Barsky <me@davidbarsky.com>