otlpmetrichttp otlptracehttp: Do not parse non-protobuf reponses

[pellared]

Fixes #4717

[codecov]

Codecov Report

Merging #4719 (46d0335) into main (d968509) will decrease coverage by 0.1%.
The diff coverage is 61.1%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4719     +/-   ##
=======================================
- Coverage   81.8%   81.8%   -0.1%     
=======================================
  Files        224     224             
  Lines      18083   18113     +30     
=======================================
+ Hits       14796   14817     +21     
- Misses      2990    3000     +10     
+ Partials     297     296      -1     

Files	Coverage Δ
exporters/otlp/otlpmetric/otlpmetrichttp/client.go	80.8% <100.0%> (+0.3%)	⬆️
...pmetric/otlpmetrichttp/internal/otest/collector.go	56.7% <100.0%> (+2.0%)	⬆️
exporters/otlp/otlptrace/otlptracehttp/client.go	76.9% <100.0%> (+0.3%)	⬆️
...pmetric/otlpmetricgrpc/internal/otest/collector.go	21.8% <0.0%> (-1.1%)	⬇️

... and 1 file with indirect coverage changes