added some documentation and a watermark gauge

[Max-Meldrum]

Missing rustdoc for PerfEvents

/// Configurable hardware counters that may be added to an Operator's config
///
///```no_run
/// let mut perf_events = PerfEvents::new();
/// perf_events.add(HardwareCounter::CpuCycles);
/// perf_events.add(HardwareCounter::BranchMisses);
///```
#[derive(Deserialize, Clone, Debug, Default)]
pub struct PerfEvents {
    pub counters: Vec<HardwareCounter>,
}

[Max-Meldrum]

Closes #259