Introduce more grpc server metrics

[zuston]

What changes were proposed in this pull request?

Introduce more grpc server metrics

1. grpc_server_executor_active_threads
2. grpc_server_executor_blocking_queue_size
3. grpc_server_connection_size

Why are the changes needed?

Now if the executors size is not enough for handling requests, there is no way to find this problem. And the connection number is need to use the linux command of nestat to find.

So this PR is to introduce these metrics.

Does this PR introduce any user-facing change?

No

How was this patch tested?

Unit Tests

[zuston]

cc @jerqi

[zuston]

If u have time, could u help check this improvement? @jerqi

[codecov-commenter]

Codecov Report

Merging #150 (ee5dc33) into master (04cbdbb) will increase coverage by 0.76%. The diff coverage is 65.85%.

@@             Coverage Diff              @@
##             master     #150      +/-   ##
============================================
+ Coverage     57.20%   57.97%   +0.76%     
- Complexity     1200     1224      +24     
============================================
  Files           150      151       +1     
  Lines          8185     8237      +52     
  Branches        773      772       -1     
============================================
+ Hits           4682     4775      +93     
+ Misses         3257     3216      -41     
  Partials        246      246              

Impacted Files	Coverage Δ
...le/common/rpc/MonitoringServerTransportFilter.java	0.00% <0.00%> (ø)
...java/org/apache/uniffle/common/rpc/GrpcServer.java	28.07% <84.21%> (+28.07%)	:arrow_up:
...org/apache/uniffle/common/metrics/GRPCMetrics.java	46.51% <84.61%> (+46.51%)	:arrow_up:
...apache/uniffle/coordinator/CoordinatorMetrics.java	93.93% <0.00%> (+0.18%)	:arrow_up:
...rg/apache/uniffle/coordinator/CoordinatorConf.java	96.39% <0.00%> (+0.24%)	:arrow_up:
...he/uniffle/server/buffer/ShuffleBufferManager.java	82.25% <0.00%> (+0.43%)	:arrow_up:
...pache/spark/shuffle/writer/WriteBufferManager.java	83.45% <0.00%> (+1.23%)	:arrow_up:
...e/uniffle/server/storage/SingleStorageManager.java	67.21% <0.00%> (+1.63%)	:arrow_up:
...ache/uniffle/coordinator/SimpleClusterManager.java	91.07% <0.00%> (+2.77%)	:arrow_up:
...apache/uniffle/coordinator/ApplicationManager.java	83.70% <0.00%> (+2.88%)	:arrow_up:
... and 3 more

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more

[zuston]

Thanks for your patient review. @jerqi