Fix duplicate label set error in certMetaCollector for failed issuances

[netops2devops]

This MR issues the fix for a bug in metrics endpoint collector that keeps track of failed certs in metrics.db and thus starting acme-proxy may leads to the following error when querying the metrics endpoint.

note - The acme-proxy server started just fine and was issuing certs.

curl http://acmeproxy-dev.example.com:9234/metrics
An error has occurred while serving metrics:

56 error(s) occurred:
* collected metric "externalcas_certificate_info" { label:{name:"common_name" value:"acmeproxy-dev.example.com"} label:{name:"issuer" value:""} label:{name:"sans" value:"acmeproxy-dev.example.com"} label:{name:"serial" value:""} label:{name:"status" value:"failure"} gauge:{value:1}} was collected before with the same name and label values
* collected metric "externalcas_certificate_signing_duration_seconds" { label:{name:"common_name" value:"acmeproxy-dev.example.com"} label:{name:"serial" value:""} label:{name:"status" value:"failure"} gauge:{value:30.938804371}} was collected before with the same name and label values

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.28%. Comparing base (5d875a0) to head (3480389).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
+ Coverage   44.17%   44.28%   +0.11%     
==========================================
  Files           6        6              
  Lines         489      490       +1     
==========================================
+ Hits          216      217       +1     
  Misses        262      262              
  Partials       11       11              

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