support url-encoded OTEL_EXPORTER_OTLP_HEADERS values

[brettmc]

Per a recent spec clarification in open-telemetry/opentelemetry-specification#3832 OTLP headers should support url-encoded header values.

Fixes: #1239

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.08%. Comparing base (b20c45d) to head (b592c58).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1242      +/-   ##
============================================
- Coverage     83.09%   83.08%   -0.02%     
+ Complexity     2275     2273       -2     
============================================
  Files           285      285              
  Lines          6460     6455       -5     
============================================
- Hits           5368     5363       -5     
  Misses         1092     1092              

Flag	Coverage Δ
7.4	81.72% <100.00%> (-0.02%)	⬇️
8.0	82.98% <100.00%> (-0.02%)	⬇️
8.1	83.14% <100.00%> (-0.02%)	⬇️
8.2	83.14% <100.00%> (-0.02%)	⬇️
8.3	83.14% <100.00%> (-0.02%)	⬇️
8.4	83.14% <100.00%> (-0.02%)	⬇️

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

Files	Coverage Δ
src/Contrib/Otlp/LogsExporterFactory.php	100.00% <100.00%> (ø)
src/Contrib/Otlp/MetricExporterFactory.php	97.61% <100.00%> (-0.16%)	⬇️
src/Contrib/Otlp/OtlpUtil.php	73.68% <100.00%> (+12.14%)	⬆️
src/Contrib/Otlp/SpanExporterFactory.php	100.00% <100.00%> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b20c45d...b592c58. Read the comment docs.