DfsPackFile: re-add metrics for bitmap index loads

The calculations for readBitmapIdxBytes and readBitmapIdxMicros
metrics were unintentionally removed in
https://gerrithub.io/c/eclipse-jgit/jgit/+/1177262, so this change
adds those metrics back.

Change-Id: I7ac7769acd092680933a27b2b3e70dd67690cfbf

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsPackFile.java

diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsPackFile.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsPackFile.java
index 845feab..48ed47a 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsPackFile.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsPackFile.java
@@ -1280,9 +1280,12 @@ private DfsBlockCache.Ref<PackObjectSizeIndex> loadObjectSizeIndex(
 	private DfsBlockCache.Ref<PackBitmapIndex> loadBitmapIndex(DfsReader ctx,
 			DfsStreamKey bitmapKey) throws IOException {
 		ctx.stats.readBitmap++;
+		long start = System.nanoTime();
 		PackBitmapIndexLoader.LoadResult result = bitmapLoader
 				.loadPackBitmapIndex(ctx, this);
 		bitmapIndex = result.bitmapIndex;
+		ctx.stats.readBitmapIdxBytes += result.bytesRead;
+		ctx.stats.readBitmapIdxMicros += elapsedMicros(start);
 		return new DfsBlockCache.Ref<>(bitmapKey, REF_POSITION,
 				result.bytesRead, result.bitmapIndex);
 	}