Discard unknown metrics in BSC (#18562)

Author: serbel324

ydb/core/mind/bscontroller/impl.h

@@ -280,6 +280,7 @@ class TBlobStorageController : public TActor<TBlobStorageController>, public TTa
             const ui64 allocatedSizeBefore = Metrics.GetAllocatedSize();
             const ui32 prevStatusFlags = Metrics.GetStatusFlags();
             Metrics.MergeFrom(vDiskMetrics);
+            Metrics.DiscardUnknownFields();
             MetricsDirty = true;
             UpdateVDiskMetrics();
             *allocatedSizeIncrementPtr = Metrics.GetAllocatedSize() - allocatedSizeBefore;