Use the same prometheus registry for both oc and otel exporters

[paivagustavo]

Description: Use the same prometheus.Registry for both OC and OTel SDK exporters.

Link to tracking Issue: #5687

Testing: Added unit test and did some manual tests with and without the featuregate to ensure that nothing is broken.

Documentation: The observed difference between both exporters is how they export Resource Attributes. OpenCensus appends resource attributes to all metrics while OpenTelemetry advocates for the use of an additional target_info metric which contains all resource attributes and other metrics are only appended with job and instance which can be derived from the service.name, service.namespace, and service.instance.id attributes.

OTel-Go doesn't implement the target_info yet, but I've added some notes and TODOs to track those.

[codecov]

Codecov Report

Base: 92.03% // Head: 91.97% // Decreases project coverage by -0.05% ⚠️

Coverage data is based on head (cc3c299) compared to base (a9f41a2).
Patch coverage: 75.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6297      +/-   ##
==========================================
- Coverage   92.03%   91.97%   -0.06%     
==========================================
  Files         218      219       +1     
  Lines       13203    13242      +39     
==========================================
+ Hits        12151    12179      +28     
- Misses        832      840       +8     
- Partials      220      223       +3     

Impacted Files	Coverage Δ
service/telemetry.go	87.57% <75.00%> (-1.46%)	⬇️
service/service.go	69.44% <0.00%> (-1.99%)	⬇️
pdata/plog/logs.go	96.00% <0.00%> (ø)
confmap/resolver.go	98.13% <0.00%> (ø)
pdata/pmetric/metrics.go	100.00% <0.00%> (ø)
pdata/plog/plogotlp/logs.go	51.06% <0.00%> (ø)
pdata/pcommon/trace_state.go	100.00% <0.00%> (ø)
pdata/ptrace/ptraceotlp/traces.go	51.06% <0.00%> (ø)
pdata/pmetric/pmetricotlp/metrics.go	55.81% <0.00%> (ø)
... and 9 more

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[bogdandrutu]

Please update descriptions, and add some unit tests

[paivagustavo]

@bogdandrutu I've added some unit tests, not sure if that was the best place, open for suggestions if you have any.

Also documented on the tests the differences between how both SDK exports Resource attributes. I'm not sure what is the best way for us to document this public-wise, since it can be considered as a breaking change for some.

[bogdandrutu]

I would prefer proper unittests, I know we have some kind of "integration" tests there, but they are actually complicated and hard to follow. I would rather remove them and have proper unit-tests.