fix: key for otlp traces protocol app configuration

[tsloughter]

Fix for #788

This was broken by a find-replace I did when breaking things up across modules. Looking at adding a regression test still.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.20%. Comparing base (eb494e8) to head (3a8cbe1).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #789   +/-   ##
=======================================
  Coverage   73.20%   73.20%           
=======================================
  Files          64       64           
  Lines        1941     1941           
=======================================
  Hits         1421     1421           
  Misses        520      520           

Flag	Coverage Δ
api	69.90% <ø> (ø)
elixir	17.32% <ø> (ø)
erlang	74.53% <ø> (ø)
exporter	73.17% <ø> (ø)
sdk	77.30% <ø> (ø)
zipkin	54.16% <ø> (ø)

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[tsloughter]

Added regression test. Verified it fails with the old code.