[Dubbo-monitor] change monitor log level

[VictorZeng]

What is the purpose of the change

fix #1439

Brief changelog

change monitor log level to Debug

Verifying this change

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[codecov-io]

Codecov Report

Merging #1443 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1443      +/-   ##
==========================================
- Coverage   31.59%   31.56%   -0.03%     
==========================================
  Files         682      682              
  Lines       33035    33034       -1     
  Branches     6592     6591       -1     
==========================================
- Hits        10436    10428       -8     
- Misses      20748    20753       +5     
- Partials     1851     1853       +2

Impacted Files	Coverage Δ
.../com/alibaba/dubbo/monitor/dubbo/DubboMonitor.java	73.83% <100%> (+0.68%)	⬆️
...ubbo/rpc/protocol/dubbo/ChannelWrappedInvoker.java	41.66% <0%> (-8.34%)	⬇️
...ba/dubbo/remoting/transport/netty/NettyServer.java	67.85% <0%> (-3.58%)	⬇️
...bo/remoting/transport/netty/NettyCodecAdapter.java	55.22% <0%> (-1.5%)	⬇️
...baba/dubbo/remoting/transport/mina/MinaClient.java	57.97% <0%> (-1.45%)	⬇️
...om/alibaba/dubbo/rpc/filter/ActiveLimitFilter.java	83.33% <0%> (ø)	⬆️

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[kimmking]

@VictorZeng :)

[ralf0131]

LGTM, thanks for the pull request!