Fix bug preventing configuring span processors

[garthk]

If I specify a configuration key under processors other than otel_batch_processor, it crashes with a function clause error. I trust that's unintentional. :)

[codecov]

Codecov Report

Merging #232 (8847e1a) into main (7a7eb9b) will decrease coverage by 0.14%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #232      +/-   ##
==========================================
- Coverage   34.87%   34.73%   -0.15%     
==========================================
  Files          37       37              
  Lines        3051     3052       +1     
==========================================
- Hits         1064     1060       -4     
- Misses       1987     1992       +5     

Flag	Coverage Δ
api	61.11% <ø> (ø)
elixir	18.05% <ø> (ø)
erlang	34.58% <0.00%> (-0.15%)	⬇️
exporter	19.48% <ø> (-0.10%)	⬇️
sdk	77.14% <0.00%> (-0.51%)	⬇️

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

Impacted Files	Coverage Δ
apps/opentelemetry/src/otel_configuration.erl	78.57% <0.00%> (-1.14%)	⬇️
apps/opentelemetry/src/otel_batch_processor.erl	66.32% <0.00%> (-2.05%)	⬇️
...ntelemetry_exporter/src/opentelemetry_exporter.erl	75.37% <0.00%> (-1.50%)	⬇️

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[tsloughter]

Yup. Thanks!