Adding last_seen back in as I don't feel like making a persisted version of the model just now

src/clients/directory/presence.rs

@@ -14,6 +14,7 @@ pub struct MixNodePresence {
     pub host: String,
     pub pub_key: String,
     pub layer: u64,
+    pub last_seen: u64,
 }
 
 #[derive(Deserialize, Serialize)]

src/clients/directory/requests/presence_mixnodes_post.rs

@@ -80,6 +80,7 @@ mod metrics_get_request {
                 host: "foo.com".to_string(),
                 pub_key: "abc".to_string(),
                 layer: 1,
+                last_seen: 0,
             }
         }
     }