update file-based configuration to allow for multiple inputs

[brettmc]

The file-configuration examples suggest that it should be possible to parse multiple configuration files:

A more complex case might consist of parsing multiple configuration files from different sources, merging them using custom logic, and creating SDK components from the merged configuration model

The OTEL_EXPERIMENTAL_CONFIG_FILE and related configuration.parse method don't allow for multiple inputs, so I propose to adjust the descriptions of these to allow for multiple inputs.

Also, in PHP SIG's implementation, when we parse config, we also apply default values which makes "custom merging" quite difficult, whereas merging the YAML inputs is straightforward. So, I've also proposed that multiple config inputs should be merged prior to parsing.

This is related to open-telemetry/opentelemetry-configuration#102 where I want to apply a custom sampler to a file-based config without breaking other SIGs by providing config they cannot parse.

For non-trivial changes, follow the change proposal process.

• Related issues #
• Related OTEP(s) #
• Links to the prototypes (when adding or changing features)
• CHANGELOG.md file updated for non-trivial changes
• spec-compliance-matrix.md updated if necessary

[arminru]

cc @open-telemetry/configuration-maintainers

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[github-actions]

Closed as inactive. Feel free to reopen if this PR is still being worked on.