Detailed validator monitoring (#2151)

## Issue Addressed

- Resolves #2064

## Proposed Changes

Adds a `ValidatorMonitor` struct which provides additional logging and Grafana metrics for specific validators.

Use `lighthouse bn --validator-monitor` to automatically enable monitoring for any validator that hits the [subnet subscription](https://ethereum.github.io/eth2.0-APIs/#/Validator/prepareBeaconCommitteeSubnet) HTTP API endpoint.

Also, use `lighthouse bn --validator-monitor-pubkeys` to supply a list of validators which will always be monitored.

See the new docs included in this PR for more info.

## TODO

- [x] Track validator balance, `slashed` status, etc.
- [x] ~~Register slashings in current epoch, not offense epoch~~
- [ ] Publish Grafana dashboard, update TODO link in docs
- [x] ~~#2130 is merged into this branch, resolve that~~

common/lighthouse_metrics/src/lib.rs

@@ -282,6 +282,12 @@ pub fn inc_counter_by(counter: &Result<IntCounter>, value: u64) {
     }
 }
 
+pub fn set_gauge_vec(int_gauge_vec: &Result<IntGaugeVec>, name: &[&str], value: i64) {
+    if let Some(gauge) = get_int_gauge(int_gauge_vec, name) {
+        gauge.set(value);
+    }
+}
+
 pub fn set_gauge(gauge: &Result<IntGauge>, value: i64) {
     if let Ok(gauge) = gauge {
         gauge.set(value);