Function phase 

pub fn phase(phase: Phase) -> PhaseGuard

Time a phase until the guard drops.

use majordomus_cli::perf::{self, Phase};
let before = perf::COUNTERS.snapshot().phases["cache_lookup"].count;
{
    let _guard = perf::phase(Phase::CacheLookup);
}
assert_eq!(perf::COUNTERS.snapshot().phases["cache_lookup"].count, before + 1);