Refine key normalization for Environment Variables Context Propagation Carrier

[pellared]

What are you trying to achieve?

Define the "key normalization" in Environment Variables as Context Propagation Carriers

Reason POSIX Standards:

Environment variable names used by the utilities in the Shell and Utilities volume of POSIX.1-2024 consist solely of uppercase letters, digits, and the ('_') from the characters defined in Portable Character Set and do not begin with a digit.

The normalization would be

• Replace all leading [0-9] with _ Add _ in front if the first character is a [0-9] digit (digits cannot be leading)
• Leave [A-Z][0-9]_
• Make [a-z] uppercase
• Replace all other characters with _

This way the env var carrier will work with all w3c propagators as well b3 propagator and possibly any other propagator unless it uses some strange characters that would lead into collision after normalization.

Related to #740

Prior art:

• envcar: add environment carrier opentelemetry-go-contrib#8442 (the closest implemntation, does not handle leading digits)
• Implement Environment Variable Context Propagation carriers in api/in… opentelemetry-java#8074 (uppercasing and replacing some characters with _)
• [carriers] add env carrier opentelemetry-python#4609 (uppercasing only)

Additional context.

I am also not sure if anyone is implementing this as a propagator wrapper (and not carrier).

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[pellared]

CC @adrielp @Bhagirath00 @Joibel @kamphaus

[pellared]

Instead of replacing leading digits with _ we can simply add an _ in front of the digits.