engine: fix divergent MVCCStats updates

Found while investigating cockroachdb#20554 (though this does not at all fix the
issue discovered there; that will be a separate PR).

We were incorrectly updating GCBytesAge when moving an intent. The old code was
pretty broken (and, with hindsight, it is still after this commit, as is exposed
by the various child commits). Its main problem was that it failed to account
for the `GCBytesAge` difference that would result from moving the intent due
to the incorrect assumption that the size of the intent would be the same. The
code was also somewhat intransparent and an attempt has been made to improve
its legibility.

This change raises the question of what to do about the divergent stats that
exist in real-world clusters. As part of addressing cockroachdb#20554, we'll need a
mechanism to correct the stats anyway, and so I will defer its introduction.

You'll want to view this diff with `?w=1` (insensitive to whitespace changes).

Release note: None.

pkg/storage/engine/mvcc.go

@@ -303,32 +303,48 @@ func updateStatsOnResolve(
 	commit bool,
 ) enginepb.MVCCStats {
 	var ms enginepb.MVCCStats
+
+	// NB: this is logging for ongoing work on #20554.
+	if false {
+		defer func() {
+			log.Infof(context.TODO(), "onResolve\n"+
+				"orig: ts=%d metaKeySize=%d metaValSize=%d KeyBytes=%d ValBytes=%d\n"+
+				"meta: ts=%d metaKeySize=%d metaValSize=%d KeyBytes=%d ValBytes=%d\n"+
+				"%+v",
+				orig.Timestamp.WallTime, origMetaKeySize, origMetaValSize, orig.KeyBytes, orig.ValBytes,
+				meta.Timestamp.WallTime, metaKeySize, metaValSize, meta.KeyBytes, meta.ValBytes,
+				&ms)
+		}()
+	}
+
 	// In this case, we're only removing the contribution from having the
 	// meta key around from orig.Timestamp to meta.Timestamp.
 	ms.AgeTo(orig.Timestamp.WallTime)
 	sys := isSysLocal(key)
 
-	// Always zero.
-	keyDiff := metaKeySize - origMetaKeySize
-	// This is going to be nonpositive: the old meta key was
-	// real, the new one is implicit.
-	valDiff := metaValSize - origMetaValSize
-
 	if sys {
-		ms.SysBytes += keyDiff + valDiff
+		ms.SysBytes += metaKeySize + metaValSize - origMetaValSize - origMetaKeySize
+		ms.AgeTo(meta.Timestamp.WallTime)
 	} else {
 		if !meta.Deleted {
-			ms.LiveBytes += keyDiff + valDiff
+			ms.LiveBytes += metaKeySize + metaValSize - origMetaValSize - origMetaKeySize
 		}
-		ms.KeyBytes += keyDiff
-		ms.ValBytes += valDiff
+		// At orig.Timestamp, the original meta key disappears.
+		ms.KeyBytes -= origMetaKeySize + orig.KeyBytes
+		ms.ValBytes -= origMetaValSize + orig.ValBytes
+
 		// If committing, subtract out intent counts.
 		if commit {
 			ms.IntentBytes -= (meta.KeyBytes + meta.ValBytes)
 			ms.IntentCount--
 		}
+
+		ms.AgeTo(meta.Timestamp.WallTime)
+
+		// At meta.Timestamp, the new meta key appears.
+		ms.KeyBytes += metaKeySize + meta.KeyBytes
+		ms.ValBytes += metaValSize + meta.ValBytes
 	}
-	ms.AgeTo(meta.Timestamp.WallTime)
 	return ms
 }