Expose the startup timestamp to Prometheus (#2233)

## Issue Addressed

Resolves #1788

## Proposed Changes

As per #1788, expose the time at which the process started via the `process_start_time_seconds` Prometheus metric. This will help users track uptime.

## Additional Info

NA

lighthouse/Cargo.toml

@@ -41,6 +41,8 @@ directory = { path = "../common/directory" }
 lighthouse_version = { path = "../common/lighthouse_version" }
 account_utils = { path = "../common/account_utils" }
 remote_signer = { "path" = "../remote_signer" }
+lighthouse_metrics = { path = "../common/lighthouse_metrics" }
+lazy_static = "1.4.0"
 
 [dev-dependencies]
 tempfile = "3.1.0"

lighthouse/src/main.rs

@@ -1,3 +1,5 @@
+mod metrics;
+
 use beacon_node::{get_eth2_network_config, ProductionBeaconNode};
 use clap::{App, Arg, ArgMatches};
 use env_logger::{Builder, Env};
@@ -219,6 +221,9 @@ fn run<E: EthSpec>(
 
     let log = environment.core_context().log().clone();
 
+    // Allow Prometheus to export the time at which the process was started.
+    metrics::expose_process_start_time(&log);
+
     if matches.is_present("spec") {
         warn!(
             log,

lighthouse/src/metrics.rs

@@ -0,0 +1,22 @@
+use lazy_static::lazy_static;
+pub use lighthouse_metrics::*;
+use slog::{error, Logger};
+use std::time::{SystemTime, UNIX_EPOCH};
+
+lazy_static! {
+    pub static ref PROCESS_START_TIME_SECONDS: Result<IntGauge> = try_create_int_gauge(
+        "process_start_time_seconds",
+        "The unix timestamp at which the process was started"
+    );
+}
+
+pub fn expose_process_start_time(log: &Logger) {
+    match SystemTime::now().duration_since(UNIX_EPOCH) {
+        Ok(duration) => set_gauge(&PROCESS_START_TIME_SECONDS, duration.as_secs() as i64),
+        Err(e) => error!(
+            log,
+            "Failed to read system time";
+            "error" => %e
+        ),
+    }
+}