/// A profiler backend.
///
/// Installs a global `tracing` subscriber so spans emitted by
/// [`profile`](crate::profile) and
/// [`profile_scope!`](crate::profile_scope) flow into the
/// backend-specific output — a Chrome-trace file on native, the
/// DevTools timeline on web.
///
/// `install` is called once per process. Drop the returned [`Guard`]
/// to flush any buffered events.
/// Opaque per-process guard. Drop to flush.