[INSTALL] Resolve dependencies in opentelemetry-cpp-config.cmake (#3094)

* Resolve dependencies in opentelemetry-cpp-config.cmake

Before this change, consumers of the exported CMake targets had to know
which dependencies are needed to get the transitive target dependencies
like `absl::*`, `gRPC::grpc++`, or `protobuf::libprotobuf`.

* Add find_dependency calls for remaining deps

Now we also resolve `OpenTracing`, and `prometheus-cpp`.

If necessary, `CURL` and `ZLIB` are also resolved.

---------

Co-authored-by: Marc Alff <marc.alff@oracle.com>
Co-authored-by: Lalit Kumar Bhasin <lalit_fin@yahoo.com>

cmake/opentelemetry-cpp-config.cmake.in

@@ -70,7 +70,39 @@ set(OPENTELEMETRY_VERSION
 
 # ##############################################################################
 
-find_package(Threads)
+include(CMakeFindDependencyMacro)
+
+find_dependency(Threads)
+
+if(@WITH_ABSEIL@)
+  find_dependency(absl)
+endif()
+
+if(@WITH_OTLP_GRPC@)
+  find_dependency(gRPC)
+endif()
+
+if("@OpenTracing_FOUND@")
+  find_dependency(OpenTracing)
+endif()
+
+if("@prometheus-cpp_FOUND@")
+  find_dependency(prometheus-cpp)
+endif()
+
+if("@Protobuf_FOUND@" OR "@PROTOBUF_FOUND@")
+  find_dependency(Protobuf)
+endif()
+
+if (@WITH_HTTP_CLIENT_CURL@ AND NOT @BUILD_SHARED_LIBS@)
+  if("@CURL_FOUND@")
+    find_dependency(CURL)
+  endif()
+
+  if("@ZLIB_FOUND@")
+    find_dependency(ZLIB)
+  endif()
+endif()
 
 set_and_check(OPENTELEMETRY_CPP_INCLUDE_DIRS "@PACKAGE_INCLUDE_INSTALL_DIR@")
 set_and_check(OPENTELEMETRY_CPP_LIBRARY_DIRS "@PACKAGE_CMAKE_INSTALL_LIBDIR@")