otelconf: add remaining unmarshaling code from initial PR

[codeboten]

Part of splitting up #8026

[codecov]

Codecov Report

❌ Patch coverage is 90.18692% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.0%. Comparing base (e29165b) to head (e4d6bc4).

Files with missing lines	Patch %	Lines
otelconf/config_json.go	86.5%	8 Missing and 8 partials ⚠️
otelconf/config_yaml.go	94.3%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #8127    +/-   ##
======================================
  Coverage   80.9%   81.0%            
======================================
  Files        194     194            
  Lines      13073   13286   +213     
======================================
+ Hits       10588   10771   +183     
- Misses      2107    2127    +20     
- Partials     378     388    +10     

Files with missing lines	Coverage Δ
otelconf/config_common.go	89.7% <100.0%> (+2.2%)	⬆️
otelconf/config_yaml.go	97.0% <94.3%> (-1.7%)	⬇️
otelconf/config_json.go	92.2% <86.5%> (-2.4%)	⬇️

... and 1 file with indirect coverage changes

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