Monitoring service api (#2251)

## Issue Addressed

N/A

## Proposed Changes

Adds a client side api for collecting system and process metrics and pushing it to a monitoring service.

common/lighthouse_metrics/src/lib.rs

@@ -59,6 +59,7 @@ use std::time::Duration;
 
 use prometheus::core::{Atomic, GenericGauge, GenericGaugeVec};
 pub use prometheus::{
+    proto::{Metric, MetricFamily, MetricType},
     Encoder, Gauge, GaugeVec, Histogram, HistogramVec, IntCounter, IntCounterVec, IntGauge,
     IntGaugeVec, Result, TextEncoder,
 };