perf(telemetry): increment the copy-forward total only when it is read

copyForwardTotal_ is a second atomic increment beside copyForwardCount_ on the
same event, kept only so a metric never goes backwards: rotate() zeroes the
per-rotation tally for its log line, which leaves that counter unusable as a
rate. Its only reader is MetricsRegistry.cpp:1153, through copyForwardTotal().

Guard the increment. During a rotation window every archive-served
non-duplicate read pays for it, and with telemetry compiled out there is
nothing to read it back.

copyForwardCount_ is untouched: rotate() exchanges it for the "copied forward N
archive-served reads" warning, which is real logging, not instrumentation. The
virtual and the member stay declared unconditionally, so the nodestore
interface has the same shape in every configuration.

src/libxrpl/nodestore/DatabaseRotatingImp.cpp

@@ -227,8 +227,13 @@ DatabaseRotatingImp::fetchNodeObject(
                     // Two counters, one event: the per-rotation tally that
                     // rotate() resets for its log line, and the monotonic total
                     // the metrics gauge reads, which must never go backwards.
+                    // Only the first is needed for the log line, so the second
+                    // costs an extra atomic increment per copy-forward and is
+                    // compiled out with its only reader.
                     copyForwardCount_.fetch_add(1, std::memory_order_relaxed);
+#ifdef XRPL_ENABLE_TELEMETRY
                     copyForwardTotal_.fetch_add(1, std::memory_order_relaxed);
+#endif
                 }
                 writable->store(nodeObject);
             }