feat: enable prewarm spawn metrics (#14229)

Author: Rjected

crates/engine/tree/src/tree/metrics.rs

@@ -70,7 +70,6 @@ pub(crate) struct BlockValidationMetrics {
     /// Trie input computation duration
     pub(crate) trie_input_duration: Gauge,
     /// Prewarm spawn duration
-    #[allow(dead_code)]
     pub(crate) prewarm_spawn_duration: Gauge,
     /// Cache saving duration
     #[allow(dead_code)]

crates/engine/tree/src/tree/mod.rs

@@ -2455,6 +2455,8 @@ where
             drop(state_root_sender);
             let elapsed = prewarm_start.elapsed();
             debug!(target: "engine::tree", ?elapsed, "Done spawning prewarm threads");
+
+            self.metrics.block_validation.prewarm_spawn_duration.set(elapsed.as_secs_f64());
         }
         trace!(target: "engine::tree", block=?block_num_hash, "Executing block");