Macro metadata

macro_rules! metadata {
    (
        name: $name:expr,
        target: $target:expr,
        level: $level:expr,
        fields: $fields:expr,
        callsite: $callsite:expr,
        kind: $kind:expr
    ) => { ... };
    (
        name: $name:expr,
        target: $target:expr,
        level: $level:expr,
        fields: $fields:expr,
        callsite: $callsite:expr,
        kind: $kind:expr,
    ) => { ... };
}

Statically constructs new span metadata.

/// For example:

use tracing_core::metadata;
use tracing_core::metadata::{Kind, Level, Metadata};
static FOO_CALLSITE: MyCallsite = MyCallsite {
    // ...
};

static FOO_METADATA: Metadata = metadata!{
    name: "foo",
    target: module_path!(),
    level: Level::DEBUG,
    fields: &["bar", "baz"],
    callsite: &FOO_CALLSITE,
    kind: Kind::SPAN,
};