[BUILD] Enable building otel-cpp extensions from main repo (open-telemetry#1937)

Author: lalitb

CMakeLists.txt

@@ -628,6 +628,8 @@ if(NOT WITH_API_ONLY)
   endif()
 endif()
 
+include(cmake/opentelemetry-build-external-component.cmake)
+
 if(OPENTELEMETRY_INSTALL)
   # Export cmake config and support find_packages(opentelemetry-cpp CONFIG)
   # Write config file for find_packages(opentelemetry-cpp CONFIG)

cmake/opentelemetry-build-external-component.cmake

@@ -0,0 +1,30 @@
+# Copyright The OpenTelemetry Authors
+# SPDX-License-Identifier: Apache-2.0
+
+# Enable building external components through otel-cpp build
+# The config options are
+#  - OPENTELEMETRY_EXTERNAL_COMPONENT_PATH - Setting local path of the external
+# component as env variable
+#  - OPENTELEMETRY_EXTERNAL_COMPONENT_URL Setting github-repo of external component
+#  as env variable
+
+
+# Add custom vendor component from local path, or GitHub repo
+if(DEFINED $ENV{OPENTELEMETRY_EXTERNAL_COMPONENT_PATH})
+  # Add custom component path to build tree and consolidate binary artifacts in
+  # current project binary output directory.
+  add_subdirectory($ENV{OPENTELEMETRY_EXTERNAL_COMPONENT_PATH}
+                   ${PROJECT_BINARY_DIR}/external)
+elseif(DEFINED $ENV{OPENTELEMETRY_EXTERNAL_COMPONENT_URL})
+  # This option requires CMake 3.11+: add standard remote repo to build tree.
+  include(FetchContent)
+  FetchContent_Declare(
+    external
+    GIT_REPOSITORY $ENV{OPENTELEMETRY_EXTERNAL_COMPONENT_URL}
+    GIT_TAG "main")
+  FetchContent_GetProperties(external)
+  if(NOT external_POPULATED)
+    FetchContent_Populate(external)
+    add_subdirectory(${external_SOURCE_DIR})
+  endif()
+endif()