only split on first leading '=' in parsing key/value list config

[tsloughter]

From comment #595 (comment)

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (dc77e6f) 38.25% compared to head (a2c40d9) 38.25%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #596   +/-   ##
=======================================
  Coverage   38.25%   38.25%           
=======================================
  Files          61       61           
  Lines        3597     3597           
=======================================
  Hits         1376     1376           
  Misses       2221     2221           

Flag	Coverage Δ
api	68.16% <ø> (ø)
elixir	18.31% <ø> (ø)
erlang	38.24% <100.00%> (ø)
exporter	8.31% <ø> (ø)
sdk	78.22% <100.00%> (ø)
zipkin	54.16% <ø> (ø)

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Impacted Files	Coverage Δ
apps/opentelemetry/src/otel_configuration.erl	79.33% <100.00%> (ø)

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