Add HTTP/JSON to the otlp exporter

[MadVikingGod]

This is the 3rd and final otlp format the spec specifies. We currently already implement grpc and HTTP/protobuf.

Usage

A new option is available for the otlphttp exporter. Configuration using WithMarshal(MarshalJSON) will result in the HTTP request using json.

This would close #1122

[codecov]

Codecov Report

Merging #1586 (bc95789) into main (62e2a0f) will increase coverage by 0.0%.
The diff coverage is 92.3%.

@@          Coverage Diff          @@
##            main   #1586   +/-   ##
=====================================
  Coverage   77.3%   77.3%           
=====================================
  Files        128     128           
  Lines       6671    6681   +10     
=====================================
+ Hits        5157    5168   +11     
+ Misses      1267    1266    -1     
  Partials     247     247           

Impacted Files	Coverage Δ
exporters/otlp/otlphttp/options.go	66.6% <66.6%> (ø)
exporters/otlp/otlphttp/driver.go	93.8% <100.0%> (+0.3%)	⬆️
exporters/otlp/otlpgrpc/connection.go	88.7% <0.0%> (+1.8%)	⬆️