Add endpoint label to metrics

[Zhang21]

Add endpoint label to metrics that can represent an address of each ProbeKind(http/tcp/...)。

Detail Issue: #368

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (1053595) 99.68% compared to head (38cbd76) 99.68%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #382   +/-   ##
=======================================
  Coverage   99.68%   99.68%           
=======================================
  Files          83       83           
  Lines        5688     5722   +34     
=======================================
+ Hits         5670     5704   +34     
  Misses         12       12           
  Partials        6        6           

Impacted Files	Coverage Δ
probe/base/base.go	100.00% <100.00%> (ø)
probe/base/metrics.go	100.00% <100.00%> (ø)
probe/http/http.go	100.00% <100.00%> (ø)
probe/http/metrics.go	100.00% <100.00%> (ø)
probe/ping/metrics.go	100.00% <100.00%> (ø)
probe/ping/ping.go	100.00% <100.00%> (ø)
probe/shell/metrics.go	100.00% <100.00%> (ø)
probe/shell/shell.go	100.00% <100.00%> (ø)
probe/ssh/metrics.go	100.00% <100.00%> (ø)
probe/ssh/ssh.go	100.00% <100.00%> (ø)
... and 2 more

... and 1 file with indirect coverage changes

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