Fix elapsed time in GPU writer debug log messages (#136692) (#136705)

Author: ChrisHegarty

x-pack/plugin/gpu/src/main/java/org/elasticsearch/xpack/gpu/codec/ES92GpuHnswVectorsWriter.java

@@ -170,7 +170,7 @@ public void flush(int maxDoc, Sorter.DocMap sortMap) throws IOException {
         } catch (Throwable t) {
             throw new IOException("Failed to flush GPU index: ", t);
         }
-        var elapsed = started - System.nanoTime();
+        var elapsed = System.nanoTime() - started;
         logger.debug("Flush total time [{}ms]", elapsed / 1_000_000.0);
     }
 
@@ -212,7 +212,7 @@ private void flushFieldsWithoutMemoryMappedFile(Sorter.DocMap sortMap) throws IO
                     }
                 }
             }
-            var elapsed = started - System.nanoTime();
+            var elapsed = System.nanoTime() - started;
             logger.debug("Flushed [{}] vectors in [{}ms]", numVectors, elapsed / 1_000_000.0);
         }
     }
@@ -545,7 +545,7 @@ public void mergeOneField(FieldInfo fieldInfo, MergeState mergeState) throws IOE
         } finally {
             deleteFilesIgnoringExceptions(mergeState.segmentInfo.dir, tempRawVectorsFileName);
         }
-        var elapsed = started - System.nanoTime();
+        var elapsed = System.nanoTime() - started;
         logger.debug("Merged [{}] vectors in [{}ms]", numVectors, elapsed / 1_000_000.0);
     }