fix: processor-level config takes precedence over top-level settings

[lazureykis]

Summary

Fixes #976.

When both traces_exporter (or bsp_* settings) and processor-level config are set, the processor-level config was silently overridden by the top-level value. For example:

application:set_env(opentelemetry, processors, [
    {otel_batch_processor, #{exporter => {my_exporter, #{}}}}
]),
application:set_env(opentelemetry, traces_exporter, {other_exporter, #{}}),

The batch processor would end up using other_exporter instead of my_exporter.

Changes

• merge_processor_config_/4: Simplified to always respect processor-level config — if a key already exists in processor opts, top-level/env values don't override it.
• transform(span_processor, batch|simple): Returns empty #{} instead of BATCH_PROCESSOR_DEFAULTS/SIMPLE_PROCESSOR_DEFAULTS. Defaults are already applied via maps:merge in merge_processor_config/4, so pre-populating was redundant and prevented the merge logic from distinguishing user-set values from defaults.
• Removed the now-unused is_default/3 helper.

New precedence order

processor opts > top-level/env config > BATCH_PROCESSOR_DEFAULTS

Tests

Added processor_exporter_not_overridden (12 assertions) and processor_config_precedence_with_os_env (4 assertions) covering:

• Processor exporter not overridden by top-level traces_exporter
• Processor none exporter preserved
• bsp_*/ssp_* settings don't override processor-level equivalents
• bsp_*/ssp_* still apply when keys are absent from processor opts
• OS env (OTEL_TRACES_EXPORTER) doesn't override processor-level exporter
• Multiple processors get independent config resolution
• Reporter's exact scenario from Batch processor app env config: user-provided exporter overridden by BATCH_PROCESSOR_DEFAULTS merge #976

[lazureykis]

@tsloughter

[tsloughter]

Sorry for the delay and thanks for this. I'm trying to be certain about this. Another issue is sadly another way of configuring things needs to get in soon and will complicate this even more. But I'll likely merge this today because off the top of my head it seems right.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 17.59%. Comparing base (a007d18) to head (4337f68).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #984   +/-   ##
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  Coverage   17.59%   17.59%           
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  Files          24       24           
  Lines         722      722           
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  Hits          127      127           
  Misses        595      595           

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elixir	17.59% <ø> (ø)

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[lazureykis]

@tsloughter Friendly ping — I think this is safe to merge independently of #769: declarative config ignores env/sys.config when a file is present, so it sits as a separate tier and doesn't touch the precedence this PR fixes. Happy to rebase if #769 lands first. Rebased on latest main.