Add Loki as a log exporter (first PR: overall structure)

[gramidt]

Description:
Allow exporting of log data to Loki.

Link to tracking Issue:
#1894

Testing:
Unit tests are included

Documentation:
The documentation includes a short description and the configuration.

[codecov]

Codecov Report

Merging #1900 (0ddec14) into master (fe1b43a) will increase coverage by 0.01%.
The diff coverage is 95.83%.

@@            Coverage Diff             @@
##           master    #1900      +/-   ##
==========================================
+ Coverage   89.90%   89.91%   +0.01%     
==========================================
  Files         380      382       +2     
  Lines       18335    18383      +48     
==========================================
+ Hits        16484    16529      +45     
- Misses       1385     1387       +2     
- Partials      466      467       +1     

Flag	Coverage Δ
integration	69.88% <ø> (+0.06%)	⬆️
unit	88.62% <95.83%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
exporter/lokiexporter/loki.go	86.66% <86.66%> (ø)
exporter/lokiexporter/factory.go	100.00% <100.00%> (ø)
processor/groupbytraceprocessor/event.go	95.96% <0.00%> (-0.81%)	⬇️

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[gramidt]

@open-telemetry/collector-contrib-approvers @anuraaga - Looking forward to working with you and getting Loki added as an exporter. Per the contributing docs, the second PR will include the implementation, and the third will add it to the binary.

[clhain]

LGTM

[gramidt]

Thank you so much for reviewing, @anuraaga! I have addressed your comments and look forward to your review.

[anuraaga]

Couple more readme nits but LGTM, thanks

[gramidt]

Thank you so much, @anuraaga!

[gramidt]

@anuraaga - Are you authorized to merge this?

[gramidt]

What are the next steps to getting this merged, @anuraaga?

[anuraaga]

@gramidt We need a maintainer to merge this after the initial review. I guess people are on holidays so we may need to wait until next week.

[gramidt]

Sounds good! Thank you again for all of your help, @anuraaga!

[bogdandrutu]

@gramidt consider to add an entry in the codeowners for this component as well as for the dependabot

[gramidt]

Thank you so much for your help in getting this merged, @bogdandrutu! I'll be sure to commit the entrees for the codeowners.