opentelemetry-process-context: implement process context publishing (OTEP-4719)

[herin049]

Description

External profilers and observability tools (such as the OpenTelemetry eBPF Profiler) operate outside the instrumented process and have no access to resource attributes configured inside OpenTelemetry SDKs. OTEP-4719 (Process Context Sharing) introduces a standard mechanism for OpenTelemetry SDKs to publish resource attributes for access by out-of-process readers

This PR introduces the opentelemetry-process-context package as a Rust backed Python extension (built with maturin/pyo3) that implements the process outlined in OTEP-4719. It publishes the SDK's resource attributes to
a memory region that any external reader with access to /proc/<pid>/maps and /proc/<pid>/mem can discover.

Python API

The introduced Python API has a very light surface area, consisting of three methods to publish, update and unpublish the process context.

def publish_context(resource: Resource) -> None: ...
def update_context(resource: Resource) -> None: ...
def unpublish_context() -> None: ...

Compatability

The original OTEP explicitly mentions that this functionality is Linux only. However, I have tried to implement this package in such a way that limited testing/development can be performed natively on any Posix compatible system (e.g. MacOS). However, the full functionality of OTEP-4719 is only available on Linux and Windows is explicitly not supported.

Feature	Linux	macOS
Backing region	memfd_create("OTEL_CTX") -> mmap(MAP_PRIVATE)	Anonymous mmap
Reader discoverability	/proc/<pid>/maps shows memfd:OTEL_CTX	Not discoverable (no /proc)
Fork protection	madvise(MADV_DONTFORK)	Skipped
Mapping name	prctl(PR_SET_VMA_ANON_NAME, "OTEL_CTX")	Skipped
Publish timestamp clock	CLOCK_BOOTTIME	CLOCK_MONOTONIC
Production use	Fully supported	Development/testing only

Fixes #5291

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

uv run tox -e py314-test-opentelemetry-process-context

TODO: Add more elaborate testing.

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated