Add Prometheus Remote Write Exporter supporting Cortex - factory and config

[huyan0]

Description:
This PR replaces the close PR #1524. Comments on that PR has been addressed here. This PR is part one of PR #1525.

This PR implements the config, config YAML, and factory structs, the prwExporter struct. It also includes a sample configuration to run this Prometheus remote write exporter. The config and factory structs are fully compatible with the Collector component interfaces.

Link to tracking Issue: #1150
Related issues are:
Metrics aggregation proposal: #1422
Prometheus exporter not functional: #1255
Related spec discussion: #731

Documentation:

• Design

cc: @huyan0 @alolita @markcartertm @sonofachamp @morigs
Request for review: @jmacd @bogdandrutu @annanay25 @gouthamve @open-telemetry/collector-approvers @open-telemetry/collector-maintainers

[huyan0]

Regarding the comments on headers in PR #1524, I couldn't find a headers field inside HTTPClientSettings to merge our headers into; would appreciate some help on that part : )

[codecov]

Codecov Report

Merging #1544 into master will increase coverage by 0.03%.
The diff coverage is 96.92%.

@@            Coverage Diff             @@
##           master    #1544      +/-   ##
==========================================
+ Coverage   91.42%   91.46%   +0.03%     
==========================================
  Files         248      250       +2     
  Lines       17219    17284      +65     
==========================================
+ Hits        15743    15808      +65     
  Misses       1064     1064              
  Partials      412      412              

Impacted Files	Coverage Δ
exporter/prometheusremotewriteexporter/factory.go	95.55% <95.55%> (ø)
exporter/prometheusremotewriteexporter/exporter.go	100.00% <100.00%> (ø)
translator/internaldata/resource_to_oc.go	87.50% <0.00%> (+2.08%)	⬆️

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[bogdandrutu]

I said that you should help us with that if you don't mind :). So please create a PR that adds Headers to the Http config :)

If you don't have time probably just file an issue and point to this file to resolve

[huyan0]

I said that you should help us with that if you don't mind :). So please create a PR that adds Headers to the Http config :)

If you don't have time probably just file an issue and point to this file to resolve

Ah I see, thanks for the clarification. I will look into that and create a PR.

[bogdandrutu]

Looks good some minor comments

[huyan0]

Hi, I have updated the documents based on comments. I will leave the headers in config for now and remove them in the HTTPClientConfig PR.