[service] Allow running the Collector without any pipelines

[evan-bradley]

Description

Adds a feature gate that allows starting the Collector without running any pipelines. This will mainly be used for OpAMP, where the Collector may need to be started with only extensions

[evan-bradley]

I decided to go with only a feature gate instead of a command line option because adding the command line option meaningfully complicates the code for what is in my opinion not sufficient gain. I think we could either:

1. Abuse the feature gate system a bit and leave this as an alpha-stage feature gate for the indefinite future.
2. Move to print warnings or similar when no pipelines are enabled. I don't think this would likely be a big deal.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.66%. Comparing base (0e9e259) to head (1e629df).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12613   +/-   ##
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  Coverage   91.66%   91.66%           
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  Files         478      478           
  Lines       26456    26456           
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  Hits        24250    24250           
  Misses       1747     1747           
  Partials      459      459           

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[yurishkuro]

why not a flag in the config?

service:
  extensions: [...]
  pipelines:
    disabled: true

This would be useful for Jaeger where jaeger-query also needs only extensions, not the pipeline. Currently we're using nop components to disable the pipelines.

[mx-psi]

why not a flag in the config?

I think this is something we can discuss before we mark this feature gate as beta. I filed #12646 to continue the discussion