[Backport 2.12] [#9936] DocDB: Increase PgClient queue length

Summary:
Increase PgClient queue length from 50 to 5000.
Configurable with gflag - pg_client_svc_queue_length.

Original commit: c1e43ee/D15995

Test Plan: Jenkins

Reviewers: bogdan

Reviewed By: bogdan

Subscribers: ybase, jhe, rahuldesirazu

Differential Revision: https://phabricator.dev.yugabyte.com/D19296

src/yb/tserver/tablet_server.cc

@@ -124,6 +124,10 @@ DEFINE_int32(ts_remote_bootstrap_svc_queue_length, 50,
              "RPC queue length for the TS remote bootstrap service");
 TAG_FLAG(ts_remote_bootstrap_svc_queue_length, advanced);
 
+DEFINE_int32(pg_client_svc_queue_length, yb::tserver::TabletServer::kDefaultSvcQueueLength,
+             "RPC queue length for the Pg Client service.");
+TAG_FLAG(pg_client_svc_queue_length, advanced);
+
 DEFINE_bool(enable_direct_local_tablet_server_call,
             true,
             "Enable direct call to local tablet server");
@@ -376,7 +380,7 @@ Status TabletServer::RegisterServices() {
                                                      std::move(forward_service)));
 
   RETURN_NOT_OK(RpcAndWebServerBase::RegisterService(
-      FLAGS_svc_queue_length_default,
+      FLAGS_pg_client_svc_queue_length,
       std::make_unique<PgClientServiceImpl>(
           tablet_manager_->client_future(), std::bind(&TabletServer::TransactionPool, this),
           metric_entity(),