Add feature flag to allow enabling otel for internal metrics

[splunkericl]

Description:
Previously, the variable to gate using open telemetry metrics was a constant and always set to false. This prevents any otel collector users to enable the feature.

This change will now use feature gate flag instead. The user can register the feature flag in their code base and enable the feature.

Link to tracking Issue:
#4694

Testing:
Added unit test to run otel collector with the flag enabled. Verify and assert the metrics endpoint.

Documentation:
feature gate ID variable documentation was added.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Eric L (22acfa2)

[bogdandrutu]

Please add a changelog entry

[codeboten]

@splunkericl please address build failure issues

[splunkericl]

@codeboten I have made a couple changes to fix lint, import order and unit test. can you run the approval workflow again?

[codecov]

Codecov Report

Merging #4912 (c9a1402) into main (52d6518) will increase coverage by 0.20%.
The diff coverage is 100.00%.

❗ Current head c9a1402 differs from pull request most recent head 406f56a. Consider uploading reports for the commit 406f56a to get more accurate results

@@            Coverage Diff             @@
##             main    #4912      +/-   ##
==========================================
+ Coverage   90.82%   91.03%   +0.20%     
==========================================
  Files         181      180       -1     
  Lines       10632    10628       -4     
==========================================
+ Hits         9657     9675      +18     
+ Misses        759      736      -23     
- Partials      216      217       +1     

Impacted Files	Coverage Δ
config/configtelemetry/configtelemetry.go	90.32% <ø> (ø)
service/telemetry.go	76.08% <100.00%> (+14.72%)	⬆️
component/exporter.go	100.00% <0.00%> (ø)
component/receiver.go	100.00% <0.00%> (ø)
component/component.go	100.00% <0.00%> (ø)
component/extension.go	100.00% <0.00%> (ø)
component/processor.go	100.00% <0.00%> (ø)
internal/internalinterface/internalinterface.go
model/pdata/logs.go	95.83% <0.00%> (+13.69%)	⬆️

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[splunkericl]

@dmitryax can you please take a look on the change?