Removed StringBuilder allocation from Base64Encoding (#5051)

* Removed `StringBuilder` allocation from `Base64Encoding`

* Update Base64Encoding.cs

* implemented feedback

Author: Aaronontheweb

src/core/Akka.API.Tests/CoreAPISpec.ApproveCore.approved.txt

@@ -4810,6 +4810,7 @@ namespace Akka.Util
     {
         public const string Base64Chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789+~";
         public static string Base64Encode(this long value) { }
+        [System.ObsoleteAttribute("Do not use. Pass in prefix as a string instead.")]
         public static System.Text.StringBuilder Base64Encode(this long value, System.Text.StringBuilder sb) { }
         public static string Base64Encode(this string s) { }
     }

src/core/Akka/Actor/ActorCell.Children.cs

@@ -135,8 +135,7 @@ private IActorRef ActorOf(Props props, string name, bool isAsync, bool isSystemS
         private string GetRandomActorName(string prefix = "$")
         {
             var id = Interlocked.Increment(ref _nextRandomNameDoNotCallMeDirectly);
-            var sb = new StringBuilder(prefix);
-            return id.Base64Encode(sb).ToString();
+            return id.Base64Encode(prefix);
         }
 
         /// <summary>

src/core/Akka/Util/Base64Encoding.cs

@@ -5,6 +5,7 @@
 // </copyright>
 //-----------------------------------------------------------------------
 
+using System;
 using System.Text;
 
 namespace Akka.Util
@@ -24,14 +25,40 @@ public static class Base64Encoding
         /// </summary>
         /// <param name="value">TBD</param>
         /// <returns>TBD</returns>
-        public static string Base64Encode(this long value) => Base64Encode(value, new StringBuilder()).ToString();
+        public static string Base64Encode(this long value)
+        {
+            return Base64Encode(value, "");
+        }
+
+        internal static string Base64Encode(this long value, string prefix)
+        {
+            // 11 is the number of characters it takes to represent long.MaxValue
+            // so we will never need a larger size for encoding longs
+            Span<char> sb = stackalloc char[11 + prefix?.Length ?? 0];
+            var spanIndex = 0;
+            if (!string.IsNullOrWhiteSpace(prefix) && prefix.Length > 0)
+            {
+                prefix.AsSpan().CopyTo(sb);
+                spanIndex = prefix.Length;
+            }
+
+            var next = value;
+            do
+            {
+                var index = (int)(next & 63);
+                sb[spanIndex++] = Base64Chars[index];
+                next = next >> 6;
+            } while (next != 0);
+            return sb.Slice(0, spanIndex).ToString();
+        }
 
         /// <summary>
         /// TBD
         /// </summary>
         /// <param name="value">TBD</param>
         /// <param name="sb">TBD</param>
         /// <returns>TBD</returns>
+        [Obsolete("Do not use. Pass in prefix as a string instead.")]
         public static StringBuilder Base64Encode(this long value, StringBuilder sb)
         {
             var next = value;