log: LogTracer checks if events are enabled

[hawkw]

Motivation

Currently, the LogTracer adapter in tokio-trace-log does not check
if an event translated from a log record would be enabled by the
current dispatcher, and always dispatches the event. This means that
the tokio-trace subscribers never have the opportunity to filter that
event.

Solution

This branch changes the LogTracer to call enabled on the current
dispatcher before creating an event. Unfortunately, we can't do this in
the log::Log::enabled impl for LogTracer, since the log::Metadata
lacks all the information required to create a tokio_trace::Metadata.

There are some potential improvements that we can make as follow-ups: in
particular, the LogTracer can keep its own cache of log records and
Interests, similar to the callsite cache, to avoid re-evaluating
filters. Additionally, if we add an Event::new (see
tokio-rs/tokio#1149), we can eliminate a redundant thread-local storage
access. I'll open issues for these changes as follow-ups.

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