otlp receiver: Fix HTTP server config source and legacy port mutation

[rmfitzpatrick]

Description:
Fixing a bug - When #4519 was added it revealed that default http endpoint usage results in the otlp receiver modifying its config and that the http server starting logic disregarded the config parameter.

Reported via signalfx/splunk-otel-collector#1064

Testing:
Added unit test

Documentation:
Updated changelog, but none needed otherwise.

[codecov]

Codecov Report

Merging #4631 (7989763) into main (60e0cf6) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #4631      +/-   ##
==========================================
+ Coverage   90.36%   90.44%   +0.07%     
==========================================
  Files         181      181              
  Lines       10598    10598              
==========================================
+ Hits         9577     9585       +8     
+ Misses        797      789       -8     
  Partials      224      224              

Impacted Files	Coverage Δ
receiver/otlpreceiver/otlp.go	81.33% <100.00%> (+5.33%)	⬆️

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[bogdandrutu]

@rmfitzpatrick why not fixing #4565?

[rmfitzpatrick]

@rmfitzpatrick why not fixing #4565?

Because it sounds like they may be removed in total in the future and I opted to fix a bug in the meantime: #4565 (comment)

[bogdandrutu]

I think the decision on that issue is that "the future" is now :)

[dmitryax]

Thanks for fixing it!

It looks good to me.

@bogdandrutu I would vote for merging it before cutting the new release this week, and we can disable the legacy endpoint after that.