Fix clippy warning: use self by value for Copy type

Change to_unix_timestamp to take self by value instead of &self
since Timestamp is Copy

src/api_client.rs

@@ -2,7 +2,6 @@ use crate::config::{AgentConfig, HeartbeatMetadata};
 use crate::metrics::Metric;
 use crate::proto::agent;
 use prost::Message;
-use serde_json::json;
 use std::time::Duration;
 use thiserror::Error;
 

src/metrics/mod.rs

@@ -21,7 +21,7 @@ impl Timestamp {
         Self::now().secs - self.secs
     }
 
-    pub fn to_unix_timestamp(&self) -> i64 {
+    pub fn to_unix_timestamp(self) -> i64 {
         self.secs
     }