fix(metrics): discard block-processing time for duplicate blocks

crates/blockchain/src/store.rs

@@ -569,7 +569,7 @@ fn on_block_core(
     signed_block: SignedBlock,
     verify: bool,
 ) -> Result<(), StoreError> {
-    let _timing = metrics::time_fork_choice_block_processing();
+    let timing = metrics::time_fork_choice_block_processing();
     let block_start = std::time::Instant::now();
 
     let block = &signed_block.message;
@@ -581,6 +581,7 @@ fn on_block_core(
         .has_state(&block_root)
         .expect("DB read should succeed")
     {
+        timing.discard();
         return Ok(());
     }
 

crates/common/metrics/src/timing.rs

@@ -5,6 +5,11 @@ use std::time::Instant;
 use crate::Histogram;
 
 /// A guard that records elapsed time to a histogram when dropped.
+///
+/// The measurement can be cancelled with [`TimingGuard::discard`], which
+/// consumes the guard without recording a sample, for a timed path that turns
+/// out to be a no-op whose duration would only skew the histogram (e.g. an
+/// idempotent early return).
 pub struct TimingGuard {
     histogram: &'static Histogram,
     start: Instant,
@@ -17,6 +22,22 @@ impl TimingGuard {
             start: Instant::now(),
         }
     }
+
+    /// Consume the guard without recording a sample.
+    ///
+    /// Use when the timed work should not contribute a sample, such as a
+    /// duplicate/idempotent request that returns early: the elapsed time is
+    /// real but recording it would skew the histogram toward near-zero.
+    ///
+    /// `TimingGuard` implements [`Drop`], so its fields cannot be moved out to
+    /// destructure it directly. Wrapping in [`std::mem::ManuallyDrop`] inhibits
+    /// the recording `Drop`; the fields are `Copy`, so they are then read out
+    /// and their copies dropped here, leaving nothing to record.
+    pub fn discard(self) {
+        let guard = std::mem::ManuallyDrop::new(self);
+        let _histogram = guard.histogram;
+        let _start = guard.start;
+    }
 }
 
 impl Drop for TimingGuard {