[docs/rfc] RFC about environment variables

[mx-psi]

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Description: Adds an RFC about how environment variable resolution should work Link to tracking Issue: Fixes #9515, relates to:

• #8215
• #8565
• #9162
• #9531
• #9532

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.70%. Comparing base (65cdb18) to head (90db1c0). Report is 37 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9854      +/-   ##
==========================================
- Coverage   91.95%   91.70%   -0.25%     
==========================================
  Files         357      358       +1     
  Lines       16501    16543      +42     
==========================================
- Hits        15173    15171       -2     
- Misses       1000     1045      +45     
+ Partials      328      327       -1     

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[mx-psi]

Remaining work

• [x] Decide on https://github.com/open-telemetry/opentelemetry-collector/pull/9854#discussion_r1561181980 by raising this on the spec and seeing if we can align on that
  • Filed open-telemetry/opentelemetry-specification/pull/4002
• [x] ~~Decide on https://github.com/open-telemetry/opentelemetry-collector/pull/9854#discussion_r1549449041. I am going to try and do a PoC of the confmap behavior to double check what makes more sense~~

[codeboten]

Remaining work

* [ ]  Decide on [[docs/rfc] RFC about environment variables #9854 (comment)](https://github.com/open-telemetry/opentelemetry-collector/pull/9854#discussion_r1561181980) by raising this on the spec and seeing if we can align on that
  
  * Filed [[configuration] Fail on unsupported identifiers opentelemetry-specification#4002](https://github.com/open-telemetry/opentelemetry-specification/pull/4002)

* [ ]  Decide on [[docs/rfc] RFC about environment variables #9854 (comment)](https://github.com/open-telemetry/opentelemetry-collector/pull/9854#discussion_r1549449041). I am going to try and do a PoC of the confmap behavior to double check what makes more sense

@mx-psi it looks like the spec issue will merge within the next 2 days, is the second issue still blocking this work?

[mx-psi]

@codeboten I haven't written a fully working PoC yet. I am fine merging this without the PoC if everybody else is okay with it (it may mean minor changes while we work on implementing this)

[codeboten]

Let's merge this and change it as needed, the spec change was merged 👍🏻