Add ability to override the service pool size [DPP-1306] (#15653)

* Add ability to override the service pool size

* don't set the value in the reference.conf

ledger/participant-integration-api/src/main/scala/platform/config/ParticipantConfig.scala

@@ -25,10 +25,12 @@ final case class ParticipantConfig(
     indexService: IndexServiceConfig = IndexServiceConfig(),
     indexer: IndexerConfig = IndexerConfig(),
     participantIdOverride: Option[Ref.ParticipantId] = None,
+    servicesThreadPoolSize: Int = ParticipantConfig.DefaultServicesThreadPoolSize,
 )
 
 object ParticipantConfig {
   def defaultIndexJdbcUrl(participantId: Ref.ParticipantId): String =
     s"jdbc:h2:mem:$participantId;db_close_delay=-1;db_close_on_exit=false"
   val DefaultParticipantId: Ref.ParticipantId = Ref.ParticipantId.assertFromString("default")
+  val DefaultServicesThreadPoolSize: Int = Runtime.getRuntime.availableProcessors()
 }

ledger/sandbox-on-x/reference.conf

@@ -572,6 +572,9 @@ ledger {
         # The size of the submission batch within parallel ingestion pipeline.
         submission-batch-size = 50
       }
+      # The size of the thread pool underpinning internal services. Used for both the ledger api as well as
+      # the read and write services. When not set, the number of processors is used instead
+      # services-thread-pool-size = 50
 
     }
   }

ledger/sandbox-on-x/src/main/scala/com/daml/ledger/sandbox/SandboxOnXRunner.scala

@@ -90,7 +90,7 @@ object SandboxOnXRunner {
       timeServiceBackendO = configAdaptor.timeServiceBackend(participantConfig.apiServer)
       (stateUpdatesFeedSink, stateUpdatesSource) <- AkkaSubmissionsBridge()
 
-      servicesThreadPoolSize = Runtime.getRuntime.availableProcessors()
+      servicesThreadPoolSize = participantConfig.servicesThreadPoolSize
       servicesExecutionContext <- buildServicesExecutionContext(metrics, servicesThreadPoolSize)
 
       buildWriteServiceLambda = buildWriteService(