[Enhancement](FE) use customized grpc threadpool to get better metric for grpc from FE to BE (#13983)

Previously in Doris FE, there is no specific thread pool for grpc-client-channel,
by default the underlying netty logic would use one dynamic unbounded cache threadpool.
The workload for this grpc threadpool is unseen.
Use ThreadpoolMgr to create one customized threadpool to get Prometheus-compatible metric data.

fe/fe-common/src/main/java/org/apache/doris/common/Config.java

@@ -1510,6 +1510,12 @@ public class Config extends ConfigBase {
     @ConfField
     public static int grpc_max_message_size_bytes = 2147483647; // 2GB
 
+    /**
+     * num of thread to handle grpc events in grpc_threadmgr
+     */
+    @ConfField
+    public static int grpc_threadmgr_threads_nums = 4096;
+
     /**
      * Used to set minimal number of replication per tablet.
      */