Apply suggestions from code review

Co-authored-by: Mazdak Farrokhzad <twingoow@gmail.com>
Signed-off-by: joshua-spacetime <josh@clockworklabs.io>

Author: joshua-spacetime

crates/core/src/subscription/module_subscription_manager.rs

@@ -1289,26 +1289,46 @@ impl SubscriptionManager {
     }
 }
 
+/// An instruction sent after a transaction, successful or not, has finished,
+/// to record the metrics of that transaction.
+///
+/// Recording metrics is done in a separate worker ([`spawn_metrics_recorder`])
+/// to unblock reducer threads so that they can process the next call faster.
 struct MetricsMessage {
+    /// The name of the reducer, when run in a reducer context.
+    /// When run in a non-reducer transaction, this will be the empty string.
     reducer: String,
+    /// The metrics for the mutable part of a transaction,
+    /// e.g., when a reducer is being processed.
     metrics_for_writer: Option<TxMetrics>,
+    /// The metrics for the read-only part of a transaction.
+    /// For a reducer call, this would be the part after committing
+    /// and where the tx has been downgraded to read-only
+    /// to e.g., process subscription updates.
     metrics_for_reader: Option<TxMetrics>,
+    /// For a successful mutable transaction,
+    /// the info on committed changes.
     tx_data: Arc<Option<TxData>>,
+    /// Cached metrics counters for each workload type.
     counters: Arc<EnumMap<WorkloadType, ExecutionCounters>>,
 }
 
+/// The handle used to send work to the tx metrics recorder.
 #[derive(Clone)]
 pub struct MetricsRecorderQueue {
     tx: mpsc::UnboundedSender<MetricsMessage>,
 }
 
 impl MetricsRecorderQueue {
+    /// Instructs the tx metrics recorder to record some tx metrics.
+    ///
+    /// See `MetricsMessage` for more details on the parameters.
     pub fn send_metrics(
         &self,
         reducer: String,
         metrics_for_writer: Option<TxMetrics>,
         metrics_for_reader: Option<TxMetrics>,
-        tx_data: Arc<Option<TxData>>,
+        tx_data: Option<Arc<TxData>>,
         counters: Arc<EnumMap<WorkloadType, ExecutionCounters>>,
     ) {
         if let Err(err) = self.tx.send(MetricsMessage {
@@ -1338,7 +1358,7 @@ pub fn spawn_metrics_recorder() -> MetricsRecorderQueue {
                 tx_metrics.report(tx_data.as_ref().as_ref(), &reducer, |wl| &counters[wl]);
             }
             if let Some(tx_metrics) = metrics_for_reader {
-                tx_metrics.report(tx_data.as_ref().as_ref(), &reducer, |wl| &counters[wl]);
+                tx_metrics.report(None, &reducer, |wl| &counters[wl]);
             }
         }
     });