[CONFIGURATION] Support setting attributes from environment variable

[dbarker]

Background

Support for substituting environment variables as attribute values is needed.

See: configuration/data-model/#environment-variable-substitution).

The common example is to set service.name from OTEL_SERVICE_NAME as below.

resource:
  attributes:
    - name: service.name
      value: ${OTEL_SERVICE_NAME}
  detection/development:
    detectors:  
       - my_detector:

The AttributeNameValue nullBehavior is to ignore unset/null values. This means if OTEL_SERVICE_NAME is not set then the parser must ignore the service.name attribute in resource.attributes.

For example: In this configuration if my_detector sets service.name and OTEL_SERVICE_NAME is also set then the attribute value from the detector is overridden with the value from OTEL_SERVICE_NAME. If the env var is not set then the attribute from the detector must be used.

Scope:

• Add environment variable substitution support for all attribute value types to the YAML parser along with support for null value detection.
• Update the ConfigurationParser and SdkBuilder to only set attrbutes that have non-null values.
• Add unit tests for parsing all attribute value types and a programmatic test for the above use case