Allocate memory up front for histogram copy

src/metric.cpp

@@ -1083,21 +1083,21 @@ compute_metric_values(ArenaAllocator &arena, const LabelSets &label_sets) {
           auto *instance = it->second;
 
           // Extract data under lock - minimize critical section
-          ArenaVector<uint64_t> counts_snapshot(&arena);
+          uint64_t *counts_snapshot = arena.allocate<uint64_t>(bucket_count);
           double sum_snapshot;
           uint64_t observations_snapshot;
 
           {
             std::lock_guard<std::mutex> lock(instance->mutex);
             for (size_t i = 0; i < instance->counts.size(); ++i) {
-              counts_snapshot.push_back(instance->counts[i]);
+              counts_snapshot[i] = instance->counts[i];
             }
             sum_snapshot = instance->sum;
             observations_snapshot = instance->observations;
           }
 
           // Add to totals
-          for (size_t i = 0; i < counts_snapshot.size(); ++i) {
+          for (size_t i = 0; i < bucket_count; ++i) {
             total_counts[i] += counts_snapshot[i];
           }
           total_sum += sum_snapshot;