feat(dfir_rs): provide DfirMetricsIntervals iterator to prevent race conditions when writing metrics (#2401)

Less confusing as `reset()` is no longer used, a la feedback
#2241 (comment)

Not actually a breaking change as the metrics API has not been published
yet

---

This PR refactors the DFIR metrics API to replace the mutable `reset()` pattern with an immutable iterator-based approach, addressing race condition concerns when writing metrics. The key change introduces `DfirMetricsIntervals`, an infinite iterator that provides snapshots of metrics for each interval.

- Replaces the `reset()` method with an infinite iterator pattern via `DfirMetricsIntervals`
- Restructures `DfirMetrics` to be a simpler snapshot struct instead of managing current/previous state
- Adds new `metrics_intervals()` method to `Dfir` for obtaining the iterator

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: MingweiSamuel

dfir_rs/src/scheduled/graph.rs

@@ -20,7 +20,7 @@ use web_time::SystemTime;
 use super::context::Context;
 use super::handoff::handoff_list::PortList;
 use super::handoff::{Handoff, HandoffMeta, TeeingHandoff};
-use super::metrics::{DfirMetrics, DfirMetricsState, InstrumentSubgraph};
+use super::metrics::{DfirMetrics, DfirMetricsIntervals, InstrumentSubgraph};
 use super::port::{RECV, RecvCtx, RecvPort, SEND, SendCtx, SendPort};
 use super::reactor::Reactor;
 use super::state::StateHandle;
@@ -41,7 +41,8 @@ pub struct Dfir<'a> {
 
     pub(super) handoffs: SlotVec<HandoffTag, HandoffData>,
 
-    metrics: Rc<DfirMetricsState>,
+    /// Live-updating DFIR runtime metrics via interior mutability.
+    metrics: Rc<DfirMetrics>,
 
     #[cfg(feature = "meta")]
     /// See [`Self::meta_graph()`].
@@ -99,7 +100,7 @@ impl Dfir<'_> {
 
         // Initialize handoff metrics struct.
         Rc::make_mut(&mut self.metrics)
-            .handoff_metrics
+            .handoffs
             .insert(new_hoff_id, Default::default());
 
         let output_port = RecvPort {
@@ -303,7 +304,7 @@ impl<'a> Dfir<'a> {
             // times before the next subgraph consumes the output; don't double count those.
             // (*) - usually... always true for Hydro-generated DFIR at least.
             for &handoff_id in sg_data.preds.iter() {
-                let handoff_metrics = &self.metrics.handoff_metrics[handoff_id];
+                let handoff_metrics = &self.metrics.handoffs[handoff_id];
                 let handoff_data = &mut self.handoffs[handoff_id];
                 let handoff_len = handoff_data.handoff.len();
                 handoff_metrics
@@ -413,7 +414,7 @@ impl<'a> Dfir<'a> {
                 tracing::info!("Running subgraph.");
                 sg_data.last_tick_run_in = Some(self.context.current_tick);
 
-                let sg_metrics = &self.metrics.subgraph_metrics[sg_id];
+                let sg_metrics = &self.metrics.subgraphs[sg_id];
                 let sg_fut =
                     Box::into_pin(sg_data.subgraph.run(&mut self.context, &mut self.handoffs));
                 // Update subgraph metrics.
@@ -450,7 +451,7 @@ impl<'a> Dfir<'a> {
                         }
                     }
                 }
-                let handoff_metrics = &self.metrics.handoff_metrics[handoff_id];
+                let handoff_metrics = &self.metrics.handoffs[handoff_id];
                 handoff_metrics.curr_items_count.set(handoff_len);
             }
 
@@ -876,7 +877,7 @@ impl<'a> Dfir<'a> {
 
         // Initialize subgraph metrics struct.
         Rc::make_mut(&mut self.metrics)
-            .subgraph_metrics
+            .subgraphs
             .insert(sg_id, Default::default());
 
         sg_id
@@ -979,7 +980,7 @@ impl<'a> Dfir<'a> {
 
         // Initialize subgraph metrics struct.
         Rc::make_mut(&mut self.metrics)
-            .subgraph_metrics
+            .subgraphs
             .insert(sg_id, Default::default());
 
         sg_id
@@ -999,7 +1000,7 @@ impl<'a> Dfir<'a> {
 
         // Initialize handoff metrics struct.
         Rc::make_mut(&mut self.metrics)
-            .handoff_metrics
+            .handoffs
             .insert(handoff_id, Default::default());
 
         // Make ports.
@@ -1063,10 +1064,21 @@ impl<'a> Dfir<'a> {
         loop_id
     }
 
-    /// Returns DFIR runtime metrics accumulated since runtime creation.
-    pub fn metrics(&self) -> DfirMetrics {
-        DfirMetrics {
-            curr: Rc::clone(&self.metrics),
+    /// Returns a referenced-counted handle to the continually-updated runtime metrics for this DFIR instance.
+    pub fn metrics(&self) -> Rc<DfirMetrics> {
+        Rc::clone(&self.metrics)
+    }
+
+    /// Returns an infinite iterator of [`DfirMetrics`], where each call to `next()` ends an interval.
+    ///
+    /// The first call to [`DfirMetricsIntervals::next`] returns metrics since this DFIR instance was created. Each
+    /// subsequent call to [`DfirMetricsIntervals::next`] returns metrics since the previous call.
+    ///
+    /// Cloning the iterator "forks" it from the original, as afterwards each iterator will return different metrics
+    /// based on when [`DfirMetricsIntervals::next`] is called.
+    pub fn metrics_intervals(&self) -> DfirMetricsIntervals {
+        DfirMetricsIntervals {
+            curr: self.metrics(),
             prev: None,
         }
     }