fix concurrentHashMap get and put in large numbers concurrency

[samuelllin]

fix concurrentHashMap get and put in large numbers concurrency when MetricFetcher handler response from client

[cdfive]

LGTM

[cdfive]

The map in handleBody method is ConcurrentHashMap, I suddenly found that it was a local variable, which declared in
fetchOnce method, fetchOnce->handleResponse->handleBody, does it affects multithreading safety and performance?

Anyway, using computeIfAbsent, it is at least a kind of optimization, which simplifies the writing method and enhances the readability.

[cdfive]

fix concurrentHashMap get and put in large numbers concurrency when MetricFetcher handler response from client

Oh, in large numbers concurrency, the size of map may be too big, computeIfAbsent is better than get and put?

If we make sure it's a local variable, how about we use LinkedHashMap instead of ConcurrentHashMap? since LinkedHashMap use default computeIfAbsent method in Map interface.

How do you think about it? @samuelxw @linlinisme @jasonjoo2010 @sczyh30

[cdfive]

Sorry, I didn't read it carefully. Although it is a local variable in the fetchonce method, it is shared by multiple threads.