apollo_network_benchmark: add seconds_since_epoch helper function

Author: sirandreww-starkware

crates/apollo_network_benchmark/src/bin/broadcast_network_stress_test_node/metrics.rs

@@ -1,5 +1,5 @@
 use std::collections::HashMap;
-use std::time::Duration;
+use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
 use apollo_metrics::define_metrics;
 use apollo_metrics::metrics::LossyIntoF64;
@@ -80,6 +80,11 @@ pub fn get_throughput(message_size_bytes: usize, heartbeat_duration: Duration) -
     tps * message_size_bytes.into_f64()
 }
 
+pub fn seconds_since_epoch() -> u64 {
+    let now = SystemTime::now();
+    now.duration_since(UNIX_EPOCH).unwrap().as_secs()
+}
+
 /// Creates barebones network metrics
 pub fn create_network_metrics() -> apollo_network::metrics::NetworkMetrics {
     // Create broadcast metrics for the stress test topic