[receiver/otlp, internal, featuregate] Add featuregate to switch to localhost defaults for server-like components

[mx-psi]

Description:

• Define component.UseLocalHostAsDefaultHost in the internal/localhostgate package.
• Define featuregate.ErrIsAlreadyRegistered error, returned by Register when a gate is already registered.
• Adds support for the localhost gate on the OTLP receiver.

This PR does not remove the current warning in any way, we can remove this separately.

Link to tracking Issue: Updates #8510

Testing: Adds unit tests

Documentation: Document on OTLP receiver template and add related logging.

[codecov]

Codecov Report

Attention: 19 lines in your changes are missing coverage. Please review.

Comparison is base (5ab066e) 90.24% compared to head (cc90a90) 90.14%.

Files	Patch %	Lines
internal/localhostgate/featuregate.go	37.03%	15 Missing and 2 partials ⚠️
config/internal/warning.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8622      +/-   ##
==========================================
- Coverage   90.24%   90.14%   -0.11%     
==========================================
  Files         344      345       +1     
  Lines       17996    18025      +29     
==========================================
+ Hits        16240    16248       +8     
- Misses       1428     1446      +18     
- Partials      328      331       +3     

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[mx-psi]

To see the usage of this API on contrib take a look at open-telemetry/opentelemetry-collector-contrib/pull/27439

[mx-psi]

Adding some people that commented on the original issue as reviewers :) PTAL!

[mx-psi]

@jpkrohling after the last commit we now warn per component and only if the endpoint is not set. The approach is similar to what I did for the logging exporter:

opentelemetry-collector/exporter/loggingexporter/config.go

Lines 41 to 42 in 5c225d9

	// warnLogLevel is set on unmarshaling to warn users about `loglevel` usage.
	warnLogLevel bool

[bogdandrutu]

I just have concern with sharing the gate (code) and then removing the code.

[mx-psi]

@bogdandrutu see #8622 (comment) for an alternative way of doing this

[mx-psi]

@bogdandrutu (and others) will merge this by Wednesday EU morning (after this week's core release) unless somebody objects before then :)