trace: Allow trace instrumentation to emit log records (#992)

## Motivation

`tokio-trace` currently offers a strategy for compatibility with the
`log` crate: its macros can be dropped in as a replacement for `log`'s
macros, and a subscriber can be used that translates trace events to log
records. However, this requires the application to be aware of
`tokio-trace` and manually set up this subscriber.

Many libraries currently emit `log` records, and would like to be able
to emit `tokio-trace` instrumentation instead. The `tokio` runtimes are
one such example. However, with the current log compatibility strategy,
replacing existing logging with trace instrumentation would break
`tokio`'s logs for any downstream user which is using only `log` and not
`tokio-trace`. It is desirable for libraries to have the option to emit
both `log` _and_ `tokio-trace` diagnostics from the same instrumentation
points.

## Solution

This branch adds a `log` feature flag to the `tokio-trace` crate, which
when set, causes `tokio-trace` instrumentation to emit log records as well
as `tokio-trace` instrumentation. 

## Notes

In order to allow spans to log their names when they are entered and 
exited even when the span is disabled, this branch adds an 
`&'static Metadata` to the `Span` type. This was previously stored in
the `Inner` type and was thus only present when the span was enabled.
This makes disabled spans one word longer, but enabled spans remain
the same size.

Fixes: #949

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

1 files changed, 1 insertions, 0 deletions

diff --git a/.cirrus.yml b/.cirrus.yml
index 053ce41e..922c5bee 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -32,6 +32,7 @@ task:
   test_script:
     - . $HOME/.cargo/env
     - cargo test --all --no-fail-fast
+    - (cd tokio-trace/test-log-support && cargo test)
     - (cd tokio-trace/test_static_max_level_features && cargo test)
     - cargo doc --all
   i686_test_script: