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Towards: open-telemetry/opentelemetry-specification#5040

The current implementation of environment variable propagation carrier is not specification compliant: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/env-carriers.md

At quick glance:

• the name normalization is different
• the env var names read from the OS are not normalized when getting the value
• caching could be done only once

Additional context

Follows #3817

OTel .NET, Go, Java implementation should be already specification compliant.

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