expvar: improve Map.addKey for large number of keys

The existing implementation has poor performance for large number of
keys because it chooses to append the new key to the sorted keys array
and sort the new array again.

The improvement tries to utilize the sorted keys array by searching the
index and doing an insertion if any. It also fixes an error that
duplicate keys may be inserted in high concurrent scenes.

Fixes #31414

src/expvar/expvar.go

@@ -141,8 +141,14 @@ func (v *Map) Init() *Map {
 func (v *Map) addKey(key string) {
 	v.keysMu.Lock()
 	defer v.keysMu.Unlock()
-	v.keys = append(v.keys, key)
-	sort.Strings(v.keys)
+
+	if i := sort.SearchStrings(v.keys, key); i >= len(v.keys) {
+		v.keys = append(v.keys, key)
+	} else if v.keys[i] != key {
+		v.keys = append(v.keys, "")
+		copy(v.keys[i+1:], v.keys[i:len(v.keys)-1])
+		v.keys[i] = key
+	}
 }
 
 func (v *Map) Get(key string) Var {

src/expvar/expvar_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
+	"math/rand"
 	"net"
 	"net/http/httptest"
 	"reflect"
@@ -350,6 +351,32 @@ func BenchmarkMapAddDifferent(b *testing.B) {
 	})
 }
 
+func BenchmarkMapAddDifferentRandom(b *testing.B) {
+	procKeys := make([][]string, runtime.GOMAXPROCS(0))
+	for i := range procKeys {
+		keys := make([]string, 100)
+		for j := range keys {
+			keys[j] = fmt.Sprint(i, rand.Int())
+		}
+		procKeys[i] = keys
+	}
+
+	b.ResetTimer()
+
+	var n int32
+	b.RunParallel(func(pb *testing.PB) {
+		i := int(atomic.AddInt32(&n, 1)-1) % len(procKeys)
+		keys := procKeys[i]
+
+		for pb.Next() {
+			m := new(Map).Init()
+			for _, k := range keys {
+				m.Add(k, 1)
+			}
+		}
+	})
+}
+
 func BenchmarkMapAddSameSteadyState(b *testing.B) {
 	m := new(Map).Init()
 	b.RunParallel(func(pb *testing.PB) {