split: init replica lb splitter with global rand

In #93838 we started initializing a new seeded rand for use in the load
based splitter's reservoir sampling algorithm. Previously, the splitter
was initialized using the global rand.

`rand.Source` heap allocates on init approximately 4kb. When initialized
per-replica, this is problematic as nodes scale to large replica counts.

This patch replaces initializing a new random source per-replica with
using the global rand instead.

resolves: #94752
resolves: #94737

Release note: None

pkg/kv/kvserver/replica_init.go

@@ -13,7 +13,6 @@ package kvserver
 import (
 	"bytes"
 	"context"
-	"math/rand"
 	"time"
 
 	"github.com/cockroachdb/cockroach/pkg/keys"
@@ -96,8 +95,7 @@ func newUnloadedReplica(
 	r.mu.stateLoader = stateloader.Make(desc.RangeID)
 	r.mu.quiescent = true
 	r.mu.conf = store.cfg.DefaultSpanConfig
-	randSource := rand.New(rand.NewSource(timeutil.Now().UnixNano()))
-	split.Init(&r.loadBasedSplitter, store.cfg.Settings, randSource, func() float64 {
+	split.Init(&r.loadBasedSplitter, store.cfg.Settings, split.GlobalRandSource(), func() float64 {
 		return float64(SplitByLoadQPSThreshold.Get(&store.cfg.Settings.SV))
 	}, func() time.Duration {
 		return kvserverbase.SplitByLoadMergeDelay.Get(&store.cfg.Settings.SV)

pkg/kv/kvserver/split/decider.go

@@ -15,6 +15,7 @@ package split
 import (
 	"context"
 	"fmt"
+	"math/rand"
 	"time"
 
 	"github.com/cockroachdb/cockroach/pkg/keys"
@@ -64,6 +65,27 @@ type RandSource interface {
 	Intn(n int) int
 }
 
+// globalRandSource implements the RandSource interface.
+type globalRandSource struct{}
+
+// Float64 returns, as a float64, a pseudo-random number in the half-open
+// interval [0.0,1.0) from the RandSource.
+func (g globalRandSource) Float64() float64 {
+	return rand.Float64()
+}
+
+// Intn returns, as an int, a non-negative pseudo-random number in the
+// half-open interval [0,n).
+func (g globalRandSource) Intn(n int) int {
+	return rand.Intn(n)
+}
+
+// GlobalRandSource returns an implementation of the RandSource interface that
+// redirects calls to the global rand.
+func GlobalRandSource() RandSource {
+	return globalRandSource{}
+}
+
 var enableUnweightedLBSplitFinder = settings.RegisterBoolSetting(
 	settings.SystemOnly,
 	"kv.unweighted_lb_split_finder.enabled",