Make metrics thread-safe

Even concurrently with calling non-const methods on the associated
ConflictSet

Bench.cpp

@@ -362,10 +362,14 @@ void benchMetrics() {
   ankerl::nanobench::Bench bench;
   ConflictSet cs{0};
 
-  bench.run("metrics", [&]() {
-    ConflictSet::MetricsV1 *m;
-    int count;
-    cs.getMetricsV1(&m, &count);
+  int count;
+  ConflictSet::MetricsV1 *m;
+  cs.getMetricsV1(&m, &count);
+  bench.batch(count);
+  bench.run("fetch metric", [&]() {
+    for (int i = 0; i < count; ++i) {
+      m[i].getValue();
+    }
   });
 }