use common/expfmt instead of local http fetch for PushGateway

[Pirionfr]

implement fetch metrics using prometheus common/expfmt instead of local http Get

[codecov]

Codecov Report

Merging #29 into master will increase coverage by 1.16%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
+ Coverage   50.00%   51.16%   +1.16%     
==========================================
  Files           5        4       -1     
  Lines         326      342      +16     
==========================================
+ Hits          163      175      +12     
- Misses        153      157       +4     
  Partials       10       10              

Impacted Files	Coverage Δ
prometheus/prometheus.go	52.38% <83.33%> (+4.42%)	⬆️
jaegertracing/jaegertracing.go	33.64% <0.00%> (-7.43%)	⬇️
jaegertracing/body_dump_response_writer.go
session/session.go	90.90% <0.00%> (+1.43%)	⬆️
casbin/casbin.go	83.33% <0.00%> (+3.33%)	⬆️

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[lammel]

Looking good. Could you rebase this PR on master for merging please.

[lammel]

Thanks for your contribution @Pirionfr .