[CMAKE] add generated protobuf headers to the opentelemetry_proto target (open-telemetry#3400)

* adds the generated protobuf includes to the opentelemetry_proto target instead of using include_directories

* add changelog entry

---------

Co-authored-by: Marc Alff <[email protected]>
Co-authored-by: Lalit Kumar Bhasin <[email protected]>

Author: 3 people

CHANGELOG.md

@@ -52,6 +52,9 @@ Increment the:
 * [BUILD] Upgrade nlohmann_json to 3.12.0
   [#3406](https://github.com/open-telemetry/opentelemetry-cpp/pull/3406)
 
+* [CMAKE] Add generated protobuf headers to the opentelemetry_proto target
+  [#3400](https://github.com/open-telemetry/opentelemetry-cpp/pull/3400)
+
 ## [1.20 2025-04-01]
 
 * [BUILD] Update opentelemetry-proto version

cmake/opentelemetry-proto.cmake

@@ -290,8 +290,6 @@ add_custom_command(
   COMMENT "[Run]: ${PROTOBUF_RUN_PROTOC_COMMAND}"
   DEPENDS ${PROTOBUF_PROTOC_EXECUTABLE})
 
-include_directories("${GENERATED_PROTOBUF_PATH}")
-
 unset(OTELCPP_PROTO_TARGET_OPTIONS)
 if(CMAKE_SYSTEM_NAME MATCHES "Windows|MinGW|WindowsStore")
   list(APPEND OTELCPP_PROTO_TARGET_OPTIONS STATIC)
@@ -315,6 +313,11 @@ add_library(
   ${METRICS_SERVICE_PB_CPP_FILE})
 set_target_version(opentelemetry_proto)
 
+target_include_directories(
+  opentelemetry_proto
+  PUBLIC "$<BUILD_INTERFACE:${GENERATED_PROTOBUF_PATH}>"
+         "$<INSTALL_INTERFACE:include>")
+
 # Disable include-what-you-use on generated code.
 set_target_properties(opentelemetry_proto PROPERTIES CXX_INCLUDE_WHAT_YOU_USE
                                                      "")