Fix: fix bug when creating metrics objects concurrently

[CoolIceV]

What this PR does:
fix bug when creating metrics objects concurrently

Which issue(s) this PR fixes:
Fixes #1953

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• All ut passed (run 'go test ./...' in project root)
• After go-fmt ed , run 'go fmt project' using goland.
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• After import formatted, (using imports-formatter to run 'imports-formatter .' in project root, to format your import blocks, mentioned in CONTRIBUTING.md above)
• Your new-created file needs to have apache license at the top, like other existed file does.
• All integration test passed. You can run integration test locally (with docker env). Clone our dubbo-go-samples project and replace the go.mod to your dubbo-go, and run 'sudo sh start_integration_test.sh' at root of samples project root. (M1 Slice is not Support)

[codecov-commenter]

Codecov Report

Merging #1961 (47f1947) into 3.0 (de6fcfb) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##              3.0    #1961      +/-   ##
==========================================
- Coverage   44.32%   44.31%   -0.02%     
==========================================
  Files         283      283              
  Lines       17067    17041      -26     
==========================================
- Hits         7565     7551      -14     
+ Misses       8696     8686      -10     
+ Partials      806      804       -2     

Impacted Files	Coverage Δ
metrics/prometheus/reporter.go	30.25% <0.00%> (+2.20%)	⬆️
metadata/report/delegate/delegate_report.go	26.49% <0.00%> (-8.61%)	⬇️
...nstance/random/random_service_instance_selector.go	66.66% <0.00%> (ø)
cluster/loadbalance/p2c/loadbalance.go	80.00% <0.00%> (+3.63%)	⬆️

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[AlexStocks]

@Mulavar pls review agaiin.