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## 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>