WIP better observability for return values (SystemTime Debug is useless)

pageserver/src/page_service.rs

@@ -2264,9 +2264,12 @@ impl PageServerHandler {
         set_tracing_field_shard_id(&timeline);
 
         let result: Option<SystemTime> = timeline
-            .lease_standby_horizon(lease_id, lsn, ctx)
-            // logging happens inside
+            .lease_standby_horizon(lease_id, lsn, ctx) // logs errors internally
             .ok();
+        debug!(
+            result = result.map(|x| chrono::DateTime::<Utc>::from(x).to_rfc3339()),
+            "result"
+        ); // XXX better observability isn't great
 
         // Encode result as Option<millis since epoch>
         let bytes = result.map(|t| {

pageserver/src/tenant/timeline.rs

@@ -1867,7 +1867,8 @@ impl Timeline {
         Ok(lease)
     }
 
-    #[instrument(skip(_ctx), ret(Debug), err(Debug))]
+    /// Logs return value at info level and errors at warn level in Debug::fmt.
+    #[instrument(skip(_ctx), ret(level = tracing::Level::INFO, Debug), err(level = tracing::Level::WARN, Debug))]
     pub(crate) fn lease_standby_horizon(
         &self,
         lease_id: String,