[ISSUE #1687] Fix the problem that the heartbeat cannot be reported

[chuntaojun]

What this PR does:

Which issue(s) this PR fixes:

Fixes #1687

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Merging #1688 (7801449) into 3.0 (6710138) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              3.0    #1688      +/-   ##
==========================================
+ Coverage   41.35%   41.37%   +0.02%     
==========================================
  Files         255      255              
  Lines       14664    14664              
==========================================
+ Hits         6064     6067       +3     
+ Misses       7901     7899       -2     
+ Partials      699      698       -1     

Impacted Files	Coverage Δ
filter/metrics/filter.go	100.00% <0.00%> (+15.00%)	⬆️

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[justxuewei]

Please perform go fmt ./... on your local environment, and push the code again to pass the CI.

[liu-song]

https://github.com/apache/dubbo-go/blob/master/registry/polaris/service_discovery.go#L97，这里log里面的 nacos 可以顺带改下