fix: enable programmatic config when environment variables are unset

[YangJonghun]

Which problem is this PR solving?

• fixes incorrect configuration priority where environment variables were overriding programmatic configuration
• aligns with OpenTelemetry JS SDK convention: programmatic config > environment variables
• resolves issue where users cannot override environment variables with explicit code configuration
• enables users to programmatically enable default-excluded instrumentations (like fs) regardless of environment variable settings

Short description of the changes

• Establish correct configuration precedence: programmatic config > environment variables > defaults
• Fix priority order: userConfig.enabled is now checked first, before any environment variable logic
• Enable flexible overrides: users can now override any environment variable setting with explicit programmatic configuration
• Maintain consistency: follows the same pattern as other OpenTelemetry JS SDK packages

Configuration Priority (after changes):

1. Programmatic config (userConfig.enabled: true/false) - highest priority, always respected
2. Environment variables (OTEL_NODE_DISABLED_INSTRUMENTATIONS, OTEL_NODE_ENABLED_INSTRUMENTATIONS) - used when programmatic config is not set
3. Default exclusions (defaultExcludedInstrumentations) - applied when neither programmatic config nor env vars are set

[maryliag]

There isn't a clear guideline of what takes precedent (env or programmatic), but in the JS SDK we have been using
programmatic > env
so to keep consistency across the SDK, I would suggest you make the changes accordingly here

[YangJonghun]

@maryliag
Thank you for pointing this out! I've updated the PR to follow the programmatic > env convention as you suggested. (e1e4e4b)
I'd appreciate it if you could review the changes again. Thanks!

[maryliag]

Thank you for the changes!

[otelbot]

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