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.

include/xrpl/nodestore/detail/DatabaseRotatingImp.h

@@ -108,6 +108,10 @@ private:
     // counter unusable as a metric: a series that drops to 0 on every swap
     // cannot be rated. This one only ever increases, so rate() over it reads
     // correctly and the two uses do not fight over one variable.
+    //
+    // Incremented only when telemetry is compiled in, since the metrics registry
+    // is its only reader; otherwise it stays 0 and the fetch path does one atomic
+    // increment less per copy-forward.
     std::atomic<std::uint64_t> copyForwardTotal_{0};
 
     std::shared_ptr<NodeObject>