Humio exporter init

[Xitric]

Description:
This is the first PR for the Humio exporter, containing readme, configuration, factory, and relevant tests.
The first series of PRs will focus on implementing exporting of traces, with logs to follow soon.

Apologies for the large PR, but I hope that the changes are sufficiently trivial.

Link to tracking Issue:
#3021

Testing:

• config_test.go: Tests that the configuration options can be correctly loaded, including defaults. Also tests configuration sanitization.
• factory-test.go: Tests that a factory only creates a new exporter (currently a stub) if provided with a valid configuration.

Documentation:
Added documentation for the purpose of the exporter and all its current configuration options. This includes Humio endpoints, ingest tokens, and storage options for Humio.

Also added examples of configuring the exporter at various degrees of detail.

Lastly, I added references to configuration options inherited from:

• HTTP Client Configuration
• TLS Configuration
• Queueing, Retry, and Timeout Configuration

[codecov]

Codecov Report

Merging #3022 (2b7631b) into main (0c198ee) will increase coverage by 0.04%.
The diff coverage is 97.33%.

@@            Coverage Diff             @@
##             main    #3022      +/-   ##
==========================================
+ Coverage   91.55%   91.59%   +0.04%     
==========================================
  Files         464      467       +3     
  Lines       22846    22921      +75     
==========================================
+ Hits        20916    20994      +78     
+ Misses       1437     1436       -1     
+ Partials      493      491       -2     

Flag	Coverage Δ
integration	68.96% <ø> (-0.07%)	⬇️
unit	90.56% <97.33%> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
exporter/humioexporter/config.go	94.11% <94.11%> (ø)
exporter/humioexporter/factory.go	100.00% <100.00%> (ø)
exporter/humioexporter/traces_exporter.go	100.00% <100.00%> (ø)
exporter/signalfxexporter/dimensions/requests.go	92.15% <0.00%> (+9.80%)	⬆️

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[Xitric]

@jpkrohling I am not trying to exert any pressure, just wanted to make sure this PR does not drown, since I can see I have made it to the second page over the weekend.

I added some placeholder tests to the unimplemented parts of the trace exporter to satisfy the code coverage check. All the checks are passing now.

[jpkrohling]

Thanks for pinging me, I would have missed it otherwise. I'll check it soon.

[jpkrohling]

Approved. I had a few comments that might be addressed in the next PR.

[Xitric]

Thank you for the review and comments. I will make sure to address them all in the next PR.

[jpkrohling]

@tigrannajaryan, @bogdandrutu would one of you please merge this one? Despite my comments, this is approved and I trust @Xitric is addressing them in the next PR.

[Xitric]

@tigrannajaryan I see there were some big changes to configurations, so I understand why this could not wait ;)

I figured that I might as well address all the comments at once. I have updated the tests as appropriate, and everything seems to be working again. Thank you for the thorough code reviews, it is greatly appreciated!