Allow specifying custom headers for Loki log output

[federicotdn]

What?

Allows users to specify custom headers for the loki log output option. For example:

--log-output=loki=http://example.org,header.X-My-Header=123,header.Authorization=mytoken

In this case, requests done to example.org will contain the HTTP headers X-My-Header: 123 and Authorization: mytoken.

Why?

See issue #3216 and backlinked issue from the Grafana private repository.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make ci-like-lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

#3216
#3224 (this PR implements the other half of #3216)

[codecov-commenter]

Codecov Report

Merging #3227 (ba783fd) into master (59d9f9b) will decrease coverage by 0.09%.
The diff coverage is 94.73%.

❗ Current head ba783fd differs from pull request most recent head 2ceea68. Consider uploading reports for the commit 2ceea68 to get more accurate results

@@            Coverage Diff             @@
##           master    #3227      +/-   ##
==========================================
- Coverage   73.18%   73.10%   -0.09%     
==========================================
  Files         259      257       -2     
  Lines       19864    19890      +26     
==========================================
+ Hits        14538    14541       +3     
- Misses       4404     4420      +16     
- Partials      922      929       +7     

Flag	Coverage Δ
ubuntu	73.10% <94.73%> (-0.03%)	⬇️
windows	?

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

Files Changed	Coverage Δ
log/loki.go	68.70% <80.00%> (+0.21%)	⬆️
output/cloud/expv2/flush.go	93.47% <94.73%> (+2.43%)	⬆️
output/cloud/expv2/output.go	74.23% <100.00%> (+1.39%)	⬆️

... and 12 files with indirect coverage changes

[mstoykov]

Thanks for the PR 🙇

LGTM in general, but some comments!

I have a single requested change.

But I also would prefer if we can get a small test that actually checks this.

The last test in loki_test.go starts a test server already so you can copy it and check the correct headers are send.