Restore executor configuration for zio-grpc

scala_zio_bench/src/main/scala/com/example/helloworld/GreeterServer.scala

@@ -1,10 +1,45 @@
 package com.example.helloworld
 
-import scalapb.zio_grpc.{ServerMain, ServiceList}
+import scalapb.zio_grpc.{Server, ServerLayer, ServerMain, ServiceList}
+import zio.ZLayer
+
+import java.util.concurrent.Executors
 
 object GreeterServer extends ServerMain {
   def services: ServiceList[Any] = ServiceList.add(GreeterImpl)
 
   // Default port is 9000
   override def port = 50051
+
+  override def serverLive: ZLayer[Any, Throwable, Server] = {
+    val sb = builder
+
+    /**
+     * Allow customization of the Executor with two environment variables:
+     *
+     * <p>
+     * <ul>
+     * <li>JVM_EXECUTOR_TYPE: direct, workStealing, single, fixed, cached</li>
+     * <li>JVM_EXECUTOR_THREADS: integer value.</li>
+     * </ul>
+     * </p>
+     *
+     * The number of Executor Threads will default to the number of
+     * availableProcessors(). Only the workStealing and fixed executors will use
+     * this value.
+     */
+    val threads = System.getenv("JVM_EXECUTOR_THREADS")
+    var i_threads = Runtime.getRuntime.availableProcessors
+    if (threads != null && !threads.isEmpty) i_threads = threads.toInt
+    val value = System.getenv.getOrDefault("JVM_EXECUTOR_TYPE", "workStealing")
+    value match {
+      case "direct" => sb.directExecutor
+      case "single" => sb.executor(Executors.newSingleThreadExecutor)
+      case "fixed" => sb.executor(Executors.newFixedThreadPool(i_threads))
+      case "workStealing" => sb.executor(Executors.newWorkStealingPool(i_threads))
+      case "cached" => sb.executor(Executors.newCachedThreadPool)
+    }
+
+    ServerLayer.fromServiceList(sb, services)
+  }
 }