feat(subscriber): use per-layer filtering

[hawkw]

Currently, the console_subscriber::build() and
console_subscriber::init() functions configure an EnvFilter that
enables the tokio and runtime targets, plus any targets configured
by RUST_LOG. This means that the tokio spans used by the console
will always be enabled at the TRACE level for all layers. This
includes the fmt layer we previously added in init.

Since this resulted in the fmt layer logs being cluttered with a bunch
of tokio=trace spans and events even when only high-level application
logs are enabled by RUST_LOG, we removed the fmt layer in PR #64.

However, tokio-rs/tracing#1523 was recently merged and [released][1],
adding support for [per-layer filtering][2] to tracing-subscriber. We
can now use the per-layer filtering API to selectively enable the
tokio/runtime spans and events at the TRACE level only for the
TasksLayer, and add a filter to the fmt layer based on RUST_LOG.
This allows us to put back the fmt layer in
console_subscriber::init.

Now, if we run the example app (which uses init) with a RUST_LOG
value that enables only the app's logs, we get nice output:

However, we can also enable the console-subscriber crate's internal
logging:

And, we can still enable tokio=trace ourselves, if we actually want
to see all the task spans and waker events:

I also added some trace! and debug! macros in examples/app.rs to
demo that console_subscriber::init() also enables logging.

Closes #76

[seanmonstar]

🔥