Support QPS throttling in NettyPerfClient

[jsjtzyy]

Added some dynamic throttling logic in NettyPerfClient to issue POST
requests at given QPS.

[justinlin-linkedin]

I don't really understand your calculation here, can you elaborate?

BTW, I think some simple function like

long rtt = 1000 / request.get() - hostSleepTime;
hostSleepTime -= rrt

might just do

[jsjtzyy]

fixed

[justinlin-linkedin]

nit: Since this function will be called within a nonblocking function, maybe we shouldn't do a thread.sleep here.
Some think like

Channel channel = ctx.Channel();
ctx.channel().eventLoop().schedule(new Runnable() {
    // send the request out.
}, hostToSleepTime.get(hostname), TimeUnit.MILLSECOND);

[jsjtzyy]

Thanks for letting me know this. Temporarily use current version, will take your suggestion in future PR after several tests.

[codecov-io]

Codecov Report

Merging #1220 into master will decrease coverage by 0.11%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master    #1220      +/-   ##
============================================
- Coverage     69.61%   69.49%   -0.12%     
+ Complexity     5510     5503       -7     
============================================
  Files           432      432              
  Lines         33744    33790      +46     
  Branches       4290     4298       +8     
============================================
- Hits          23492    23484       -8     
- Misses         9078     9124      +46     
- Partials       1174     1182       +8

Impacted Files	Coverage Δ	Complexity Δ
....github.ambry/tools/perf/rest/NettyPerfClient.java	0% <0%> (ø)	0 <0> (ø)	⬇️
.../main/java/com.github.ambry.store/ScanResults.java	81.25% <0%> (-1.57%)	16% <0%> (-1%)
.../java/com.github.ambry.router/DeleteOperation.java	92.19% <0%> (-1.42%)	48% <0%> (-1%)
...va/com.github.ambry.replication/ReplicaThread.java	74.62% <0%> (-0.57%)	66% <0%> (-2%)
...ain/java/com.github.ambry.router/PutOperation.java	90.61% <0%> (-0.54%)	111% <0%> (-1%)
...in/java/com.github.ambry.store/BlobStoreStats.java	71.54% <0%> (-0.42%)	103% <0%> (-1%)
...src/main/java/com.github.ambry.commons/BlobId.java	93.18% <0%> (-0.36%)	71% <0%> (-1%)
...b.ambry.network/BlockingChannelConnectionPool.java	72.3% <0%> (+1.87%)	8% <0%> (ø)	⬇️

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