storage: make maxItersBeforeSeek metamorphic

To prevent maxItersBeforeSeek from hidding inconsistencies between seeking and
iterating in the future, the commit makes the value metamorphic, with a range of
[0,3). Going forward, unit tests will use different values for this heuristic.

Release justification: None. Not intended for v22.1.

pkg/storage/pebble_mvcc_scanner.go

@@ -21,18 +21,22 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/uncertainty"
 	"github.com/cockroachdb/cockroach/pkg/roachpb"
 	"github.com/cockroachdb/cockroach/pkg/storage/enginepb"
+	"github.com/cockroachdb/cockroach/pkg/util"
 	"github.com/cockroachdb/cockroach/pkg/util/hlc"
 	"github.com/cockroachdb/cockroach/pkg/util/mon"
 	"github.com/cockroachdb/cockroach/pkg/util/protoutil"
 	"github.com/cockroachdb/errors"
 	"github.com/cockroachdb/pebble"
 )
 
-const (
-	maxItersBeforeSeek = 0
+// Key value lengths take up 8 bytes (2 x Uint32).
+const kvLenSize = 8
 
-	// Key value lengths take up 8 bytes (2 x Uint32).
-	kvLenSize = 8
+var maxItersBeforeSeek = util.ConstantWithMetamorphicTestRange(
+	"mvcc-max-iters-before-seek",
+	10, /* defaultValue */
+	0,  /* min */
+	3,  /* max */
 )
 
 // Struct to store MVCCScan / MVCCGet in the same binary format as that