Support URL-encoded values for OTEL_EXPORTER_OTLP_HEADERS

[pyohannes]

Fixes #1577

Changes

This PR adds support for URL-encoded values in OTEL_EXPORTER_OTLP_HEADERS and related headers, which is required by the specification.

For backwards compatibility, both URL-encoded and non-URL-encoded values are supported, for example:

OTEL_EXPORTER_OTLP_HEADERS="Authorization=Basic%20AAA"
OTEL_EXPORTER_OTLP_HEADERS="Authorization=Basic AAA"

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 92.53731% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 67.3%. Comparing base (eb1a09b) to head (1904551).
Report is 3 commits behind head on main.

Files	Patch %	Lines
opentelemetry-otlp/src/exporter/http/mod.rs	16.6%	5 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1578   +/-   ##
=====================================
  Coverage   67.3%   67.3%           
=====================================
  Files        138     138           
  Lines      19355   19414   +59     
=====================================
+ Hits       13029   13084   +55     
- Misses      6326    6330    +4     

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[lalitb]

This may not be required/specified in the specs, but for consistency should we also support the URL encoded header values specified as config:

opentelemetry-rust/opentelemetry-otlp/src/exporter/http/mod.rs

Line 120 in ab9415a

pub fn with_headers(mut self, headers: HashMap<String, String>) -> Self {

[pyohannes]

This may not be required/specified in the specs, but for consistency should we also support the URL encoded header values specified as config:

I think that makes sense, I added support for decoding URL-encoded values also to with_headers.

[lalitb]

Thanks for the fix.