Support http/json in otlpmetrichttp

[petern48]

Add support for encoding in protojson format and make it configurable, keeping proto as the default option.

closes #7803

This is needed for the following:

open-telemetry/opentelemetry-go-contrib#6339
open-telemetry/opentelemetry-go-contrib#8399

[codecov]

Codecov Report

❌ Patch coverage is 38.46154% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.4%. Comparing base (34cd3a7) to head (9c18965).

Files with missing lines	Patch %	Lines
exporters/otlp/otlpmetric/otlpmetrichttp/client.go	36.8%	0 Missing and 12 partials ⚠️
...pmetric/otlpmetricgrpc/internal/otest/collector.go	0.0%	8 Missing ⚠️
...pmetric/otlpmetrichttp/internal/otest/collector.go	12.5%	1 Missing and 6 partials ⚠️
exporters/otlp/otlpmetric/otlpmetrichttp/config.go	40.0%	3 Missing ⚠️
...tlpmetric/otlpmetricgrpc/internal/oconf/options.go	83.3%	0 Missing and 1 partial ⚠️
...tlpmetric/otlpmetrichttp/internal/oconf/options.go	83.3%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7804     +/-   ##
=======================================
- Coverage   86.1%   85.4%   -0.8%     
=======================================
  Files        302     302             
  Lines      22046   22106     +60     
=======================================
- Hits       18989   18883    -106     
- Misses      2676    2702     +26     
- Partials     381     521    +140     

Files with missing lines	Coverage Δ
...tlpmetric/otlpmetricgrpc/internal/oconf/options.go	77.9% <83.3%> (-11.9%)	⬇️
...tlpmetric/otlpmetrichttp/internal/oconf/options.go	80.3% <83.3%> (-12.7%)	⬇️
exporters/otlp/otlpmetric/otlpmetrichttp/config.go	78.9% <40.0%> (-9.0%)	⬇️
...pmetric/otlpmetrichttp/internal/otest/collector.go	39.2% <12.5%> (-21.4%)	⬇️
...pmetric/otlpmetricgrpc/internal/otest/collector.go	16.5% <0.0%> (-6.2%)	⬇️
exporters/otlp/otlpmetric/otlpmetrichttp/client.go	55.6% <36.8%> (-25.2%)	⬇️

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[petern48]

The check-links job is failing due to links that were already in the .tmpl files.
https://github.com/open-telemetry/opentelemetry-go/actions/runs/21115879038/job/60721331105?pr=7804

The failing codecov CI makes sense to me as well. The modifications to collector.go are tested (and needed) in client_test.go, but codecov just doesn't detect that.

I'm just going to leave the PR here and await feedback.