Copy expvar metrics implementation from jaeger-lib

[yurishkuro]

Follow-up to #3771

Copy expvar metrics implementation from jaeger-lib, mostly removing the dependency on the lib, aside from the usage with jaegr-client-go. The implementation still depends on go-kit, where the histogram part is complicated (although could be re-implemented by using VividCortex/histogram library directly).

This also upgrades go-kit to 0.12

Partially addresses #3766

[codecov]

Codecov Report

Merging #3772 (c15f82e) into main (19e4544) will increase coverage by 0.01%.
The diff coverage is 97.67%.

@@            Coverage Diff             @@
##             main    #3772      +/-   ##
==========================================
+ Coverage   97.57%   97.59%   +0.01%     
==========================================
  Files         287      288       +1     
  Lines       16672    16765      +93     
==========================================
+ Hits        16267    16361      +94     
+ Misses        321      319       -2     
- Partials       84       85       +1     

Impacted Files	Coverage Δ
internal/metrics/expvar/factory.go	95.77% <95.71%> (+95.77%)	⬆️
internal/metrics/expvar/cache.go	100.00% <100.00%> (ø)
internal/metrics/expvar/metrics.go	100.00% <100.00%> (ø)
pkg/config/tlscfg/cert_watcher.go	94.68% <0.00%> (+2.12%)	⬆️

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