storage: avoid value retrieval of sys keys during ComputeStats

With the introduction of value separation (and especially with #149712) raft
log values may be separated into blob files to reduce write amplification. This
makes value retrieval more expensive, potentially requiring multiple additional
read I/Os.

When evaluating a raft log truncation, we use ComputeStats to compute an
MVCCStats delta for the truncation. Previously during this ComputeStats call,
we'd unnecessarily retrieve and unmarshal the values of the raft log keys. This
commit refactors this to move the value retrieval and unmarshal to only where
it's required.

Informs #149712.

Epic: none
Release note: none

Author: jbowens

pkg/storage/mvcc.go

@@ -7306,6 +7306,18 @@ func computeStatsForIterWithVisitors(
 	rangeKeyVisitor func(MVCCRangeKeyValue) error,
 ) (enginepb.MVCCStats, error) {
 	var ms enginepb.MVCCStats
+	// meta is used to store the MVCCMetadata for the current key but is only
+	// reset and initialized for a subset of keys. Specifically, meta gets
+	// initialized below when:
+	//
+	// implicitMeta=true [isValue=true && key != prevKey]: When we encounter a
+	// key that a) has a non-empty timestamp and b) is a new user key, its
+	// MVCCMetadata is implicit. The loop below will reset meta and synthesize
+	// its fields.
+	//
+	// isValue=false && !isSys: When we encounter a key that has a zero
+	// timestamp and it's not a system key, we read and unmarshal the value into
+	// the MVCCMetadata struct.
 	var meta enginepb.MVCCMetadata
 	var prevKey roachpb.Key
 	var first bool
@@ -7461,23 +7473,23 @@ func computeStatsForIterWithVisitors(
 			totalBytes := metaKeySize + metaValSize
 			first = true
 
-			if !implicitMeta {
-				v, err := iter.UnsafeValue()
-				if err != nil {
-					return enginepb.MVCCStats{}, err
-				}
-				if err := protoutil.Unmarshal(v, &meta); err != nil {
-					return ms, errors.Wrap(err, "unable to decode MVCCMetadata")
-				}
-			}
-
 			if isSys {
 				ms.SysBytes += totalBytes
 				ms.SysCount++
 				if isAbortSpanKey(unsafeKey.Key) {
 					ms.AbortSpanBytes += totalBytes
 				}
 			} else {
+				if !implicitMeta {
+					v, err := iter.UnsafeValue()
+					if err != nil {
+						return enginepb.MVCCStats{}, err
+					}
+					if err := protoutil.Unmarshal(v, &meta); err != nil {
+						return ms, errors.Wrap(err, "unable to decode MVCCMetadata")
+					}
+				}
+
 				if meta.Deleted {
 					// First value is deleted, so it's GC'able; add meta key & value bytes to age stat.
 					ms.GCBytesAge += totalBytes * (nowNanos/1e9 - meta.Timestamp.WallTime/1e9)