Add experimental C/C++ implementation for process context OTEP

This PR adds an experimental C/C++ implementation for the "Process
Context" OTEP being proposed in
open-telemetry/opentelemetry-specification#4719

This implementation previously lived in
https://github.com/ivoanjo/proc-level-demo/tree/main/anonmapping-clib
and as discussed during the OTEL profiling SIG meeting we want to add
it to this repository so it becomes easier to find and contribute to.

I've made sure to include a README explaining how to use it. Here's
the ultra-quick start (Linux-only):

```bash
$ ./build.sh
$ ./build/example_ctx --keep-running
Published: service=my-service, instance=123d8444-2c7e-46e3-89f6-6217880f7123, env=prod, version=4.5.6, sdk=example_ctx.c/c/1.2.3, resources=resource.key1=resource.value1,resource.key2=resource.value2
Continuing forever, to exit press ctrl+c...
TIP: You can now `sudo ./otel_process_ctx_dump.sh 267023` to see the context

 # In another shell
$ sudo ./otel_process_ctx_dump.sh 267023 # Update this to match the PID from above
Found OTEL context for PID 267023
Start address: 756f28ce1000
00000000  4f 54 45 4c 5f 43 54 58  02 00 00 00 0b 68 55 47  |OTEL_CTX.....hUG|
00000010  70 24 7d 18 50 01 00 00  a0 82 6d 7e 6a 5f 00 00  |p$}.P.....m~j_..|
00000020
Parsed struct:
  otel_process_ctx_signature       : "OTEL_CTX"
  otel_process_ctx_version         : 2
  otel_process_ctx_published_at_ns : 1764606693650819083 (2025-12-01 16:31:33 GMT)
  otel_process_payload_size        : 336
  otel_process_payload             : 0x00005f6a7e6d82a0
Payload dump (336 bytes):
00000000  0a 25 0a 1b 64 65 70 6c  6f 79 6d 65 6e 74 2e 65  |.%..deployment.e|
00000010  6e 76 69 72 6f 6e 6d 65  6e 74 2e 6e 61 6d 65 12  |nvironment.name.|
...
Protobuf decode:
attributes {
  key: "deployment.environment.name"
  value {
    string_value: "prod"
  }
}
attributes {
  key: "service.instance.id"
  value {
    string_value: "123d8444-2c7e-46e3-89f6-6217880f7123"
  }
}
attributes {
  key: "service.name"
  value {
    string_value: "my-service"
  }
}
...
```

Note that because the upstream OTEP is still under discussion, this
implementation is experimental and may need changes to match up with
the final version of the OTEP.

Author: ivoanjo

process-context/README.md

@@ -0,0 +1,5 @@
+# OpenTelemetry Process Context (Under development)
+
+This folder contains experimental implementations for the specification proposed in https://github.com/open-telemetry/opentelemetry-specification/pull/4719 .
+
+The process context OTEP introduces a standard mechanism for OpenTelemetry SDKs to publish process-level resource attributes for access by out-of-process readers such as the OpenTelemetry eBPF Profiler.

process-context/c-and-cpp/.gitignore

@@ -0,0 +1 @@
+build/

process-context/c-and-cpp/CMakeLists.txt

@@ -0,0 +1,110 @@
+cmake_minimum_required(VERSION 3.10)
+project(otel_process_ctx C CXX)
+
+# Set C standard
+set(CMAKE_C_STANDARD 11)
+set(CMAKE_C_STANDARD_REQUIRED ON)
+
+# Set C++ standard for C++ variants
+set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+# Note: We build here with both a C compiler as well as a C++ compiler because this implementation is expected to be
+# used with either, and we want to make sure we build cleanly on either (so we don't accidentally add C-isms that C++
+# doesn't like, or vice-versa)
+
+# Compiler flags
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wextra -O2 -fPIC -ggdb3")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -O2 -fPIC -ggdb3")
+
+# Source files
+set(SOURCES otel_process_ctx.c)
+set(SOURCES_CXX otel_process_ctx.cpp)
+
+# Create shared library
+add_library(otel_process_ctx SHARED ${SOURCES})
+set_target_properties(otel_process_ctx PROPERTIES
+    OUTPUT_NAME "otel_process_ctx"
+)
+
+# Create static library
+add_library(otel_process_ctx_static STATIC ${SOURCES})
+set_target_properties(otel_process_ctx_static PROPERTIES
+    OUTPUT_NAME "otel_process_ctx"
+)
+
+# Create shared noop library
+add_library(otel_process_ctx_noop SHARED ${SOURCES})
+set_target_properties(otel_process_ctx_noop PROPERTIES
+    OUTPUT_NAME "otel_process_ctx_noop"
+)
+target_compile_definitions(otel_process_ctx_noop PRIVATE OTEL_PROCESS_CTX_NOOP=1)
+
+# Create static noop library
+add_library(otel_process_ctx_noop_static STATIC ${SOURCES})
+set_target_properties(otel_process_ctx_noop_static PROPERTIES
+    OUTPUT_NAME "otel_process_ctx_noop"
+)
+target_compile_definitions(otel_process_ctx_noop_static PRIVATE OTEL_PROCESS_CTX_NOOP=1)
+
+# Create C++ shared library
+add_library(otel_process_ctx_cpp SHARED ${SOURCES_CXX})
+set_target_properties(otel_process_ctx_cpp PROPERTIES
+    OUTPUT_NAME "otel_process_ctx_cpp"
+    CXX_STANDARD 11
+    CXX_STANDARD_REQUIRED ON
+)
+
+# Create C++ static library
+add_library(otel_process_ctx_cpp_static STATIC ${SOURCES_CXX})
+set_target_properties(otel_process_ctx_cpp_static PROPERTIES
+    OUTPUT_NAME "otel_process_ctx_cpp"
+    CXX_STANDARD 11
+    CXX_STANDARD_REQUIRED ON
+)
+
+# Create C++ shared noop library
+add_library(otel_process_ctx_cpp_noop SHARED ${SOURCES_CXX})
+set_target_properties(otel_process_ctx_cpp_noop PROPERTIES
+    OUTPUT_NAME "otel_process_ctx_cpp_noop"
+    CXX_STANDARD 11
+    CXX_STANDARD_REQUIRED ON
+)
+target_compile_definitions(otel_process_ctx_cpp_noop PRIVATE OTEL_PROCESS_CTX_NOOP=1)
+
+# Create C++ static noop library
+add_library(otel_process_ctx_cpp_noop_static STATIC ${SOURCES_CXX})
+set_target_properties(otel_process_ctx_cpp_noop_static PROPERTIES
+    OUTPUT_NAME "otel_process_ctx_cpp_noop"
+    CXX_STANDARD 11
+    CXX_STANDARD_REQUIRED ON
+)
+target_compile_definitions(otel_process_ctx_cpp_noop_static PRIVATE OTEL_PROCESS_CTX_NOOP=1)
+
+# Set include directories
+target_include_directories(otel_process_ctx PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+target_include_directories(otel_process_ctx_static PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+target_include_directories(otel_process_ctx_noop PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+target_include_directories(otel_process_ctx_noop_static PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+target_include_directories(otel_process_ctx_cpp PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+target_include_directories(otel_process_ctx_cpp_static PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+target_include_directories(otel_process_ctx_cpp_noop PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+target_include_directories(otel_process_ctx_cpp_noop_static PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+
+# Create example executable
+add_executable(example_ctx example_ctx.c)
+target_link_libraries(example_ctx otel_process_ctx)
+
+# Create example noop executable
+add_executable(example_ctx_noop example_ctx.c)
+target_link_libraries(example_ctx_noop otel_process_ctx_noop)
+
+# Install rules
+install(TARGETS otel_process_ctx otel_process_ctx_static otel_process_ctx_noop otel_process_ctx_noop_static
+                otel_process_ctx_cpp otel_process_ctx_cpp_static otel_process_ctx_cpp_noop otel_process_ctx_cpp_noop_static
+                example_ctx example_ctx_noop
+    LIBRARY DESTINATION lib
+    ARCHIVE DESTINATION lib
+    RUNTIME DESTINATION bin
+)
+install(FILES otel_process_ctx.h DESTINATION include)