directory: adding a PersistedMixMetric model (has a timestamp)

src/clients/directory/metrics.rs

@@ -1,6 +1,15 @@
 use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
 
+#[derive(Deserialize, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct PersistedMixMetric {
+    pub pub_key: String,
+    pub received: u64,
+    pub sent: HashMap<String, u64>,
+    pub timestamp: u64,
+}
+
 #[derive(Deserialize, Serialize)]
 #[serde(rename_all = "camelCase")]
 pub struct MixMetric {