Function timed 

pub async fn timed<T, E, F>(
    metrics: &RepoMetrics,
    backend: Backend,
    op: &'static str,
    call: F,
) -> Result<T, E>
where
    F: Future<Output = Result<T, E>>,

Time one repo call and record it.

The single instrumentation point for both backends. Anything measured elsewhere would be measuring a different boundary, and the comparison would be between the timers rather than the implementations.