Function layer 

pub fn layer<W, F>(
    filter: F,
    make_writer: W,
) -> Box<dyn Layer<Registry> + Send + Sync>
where
    W: for<'writer> MakeWriter<'writer> + Send + Sync + 'static,
    F: Filter<Registry> + Send + Sync + 'static,

Build a stdout fmt layer that emits CloudLogging JSON, filtered by filter.

make_writer is the sink. Production code passes std::io::stdout, and tests pass a capturing buffer.

filter is any per-layer [Filter], not only an EnvFilter: a service that retargets its log level at runtime passes a filter it can swap out from under this layer.

The function returns a boxed layer over a [Registry], so call sites compose it with tracing_subscriber::registry().with(...).