[feature request] Support the environment variable propagation spec in cpp

[adrielp]

Related Problems?

With the environment variable context propagation spec being added, we'd like to have its support added to CPP. This is part of a larger effort to adding support in each of the currently supported languages.

Describe the solution you'd like:

I'd like the specification implemented in CPP.

Considered Alternatives

n/a

Additional Context

No response

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[ShikharShivraj]

Hi, I'd like to take this issue if possible

[perhapsmaple]

@marcalff

I have a system where this will be very helpful. Currently, I pass traceparent and tracestate as optional arguments for each of my programs spawned.

I have written a basic implementation here: #3834
with inspiration from the go PR: open-telemetry/opentelemetry-go-contrib#8442

I need help with a few clarifications:

• I have followed the Go implementation of having a single class. The Python implementation on the other hand uses two separate classes: [carriers] add env carrier opentelemetry-python#4609
• Is the raw pointer correct for this use case? From my view, the carrier shouldn't own the map. Should we use nostd::shared_ptr for safety?
• I believe Windows has a character limit. Do we need to handle this?
• Keys() only returns known OTel variables (traceparent, tracestate, baggage), not all environment variables. The Go Implementation returns all environment variables.
• Carrier is NOT thread-safe. Do we need to handle this?