http fixes, pushgateway client

[gistart]

• http: ensure data ends with "\n\n"
• openmetrics: data type metric header
• added openmetrics http clients for pushgateway / prom-aggregation-gateway

[codecov]

Codecov Report

Merging #2 (d821f10) into master (73abc55) will decrease coverage by 0.80%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
- Coverage   97.89%   97.08%   -0.81%     
==========================================
  Files          14       14              
  Lines         427      446      +19     
  Branches       47       48       +1     
==========================================
+ Hits          418      433      +15     
- Misses          4        8       +4     
  Partials        5        5              

Impacted Files	Coverage Δ
prometheus_push_client/__init__.py	100.00% <ø> (ø)
prometheus_push_client/decorators.py	94.87% <75.00%> (-2.36%)	⬇️
prometheus_push_client/formats/base.py	88.46% <83.33%> (-6.28%)	⬇️
prometheus_push_client/formats/openmetrics.py	91.66% <100.00%> (+0.75%)	⬆️
prometheus_push_client/transports/http.py	100.00% <100.00%> (ø)
prometheus_push_client/version.py	100.00% <100.00%> (ø)

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