kvflowcontrol: re-enable kvadmission.flow_control.mode metamorphism

The cluster setting `kvadmission.flow_control.mode` is used to control
whether replication admission control applies to only elastic work, or
applies to all work which sets an admission priority.

The cluster setting was prior to cockroachdb#132125, metamorphic in tests,
selecting either value with equal probability. cockroachdb#132125 disabled the
metamorphism in order to prevent the newly introduced send queue / pull
mode from being run unexpectedly, while still under testing.

With testing now in place, re-enable the metamorphism, leaving the
default value unchanged.

Resolves: cockroachdb#132364
Release note: None

pkg/kv/kvserver/kvflowcontrol/BUILD.bazel

@@ -18,6 +18,7 @@ go_library(
         "//pkg/settings",
         "//pkg/settings/cluster",
         "//pkg/util/admission/admissionpb",
+        "//pkg/util/metamorphic",
         "@com_github_cockroachdb_redact//:redact",
         "@com_github_dustin_go_humanize//:go-humanize",
     ],

pkg/kv/kvserver/kvflowcontrol/kvflowcontrol.go

@@ -19,6 +19,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/settings"
 	"github.com/cockroachdb/cockroach/pkg/settings/cluster"
 	"github.com/cockroachdb/cockroach/pkg/util/admission/admissionpb"
+	"github.com/cockroachdb/cockroach/pkg/util/metamorphic"
 	"github.com/cockroachdb/redact"
 	"github.com/dustin/go-humanize"
 )
@@ -41,7 +42,11 @@ var Mode = settings.RegisterEnumSetting(
 	settings.SystemOnly,
 	"kvadmission.flow_control.mode",
 	"determines the 'mode' of flow control we use for replication traffic in KV, if enabled",
-	modeDict[ApplyToElastic], /* default value */
+	metamorphic.ConstantWithTestChoice(
+		"kvadmission.flow_control.mode",
+		modeDict[ApplyToElastic], /* default value */
+		modeDict[ApplyToAll],     /* other value */
+	),
 	modeDict,
 )