[confmap] Fix bug where expand didn't honor escaping

[TylerHelmuth]

Description

When we promoted confmap.unifyEnvVarExpansion to beta, we found that the new expansion logic in confmap wasn't handling escaping of $$ like it is supposed to. This PR fixes that bug, but adding escaping logic for $$.

@azunna1 this fixes the bug you mentioned in #10435 around the metricstransformprocessor:

  metricstransform:
    transforms:
      - include: '^k8s\.(.*)\.(.*)$$'
        match_type: regexp
        action: update
        new_name: 'kubernetes.$${1}.$${2}'
      - include: '^container_([0-9A-Za-z]+)_([0-9A-Za-z]+)_.*'
        match_type: regexp
        action: update
        new_name: 'container.$${1}.$${2}'

Testing

Added new unit tests explicitly for escaping logic

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.35%. Comparing base (8de544b) to head (b60ceb9).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10560      +/-   ##
==========================================
+ Coverage   92.33%   92.35%   +0.01%     
==========================================
  Files         393      395       +2     
  Lines       18647    18682      +35     
==========================================
+ Hits        17218    17253      +35     
  Misses       1069     1069              
  Partials      360      360              

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[danelson]

@TylerHelmuth would you happen to know how I can identify if I am affected by this? I assumed something like the following would be problematic but it doesn't appear to be

transform/redact_sensitive_information:
  error_mode: ignore
  log_statements:
    - context: log
      statements:
        - replace_all_patterns(attributes, "value", "Bearer ey.*", "$$1<redacted>")
        - replace_pattern(body, "Bearer ey.*", "$$1<redacted>")

I have quite a few more uses of $$ being used in the transform processor and I was hoping to prove I wasn't affected. Worst case I guess I can just toggle off confmap.unifyEnvVarExpansion.

[dmitryax]

@danelson you likely will be affected. See #10713