Add CMake support for prometheus exporter (#249)

This resolved only part of #249, because the 
build rule for prometheus_test_server is not added yet.

Author: StevenYCChou

cmake/OpenCensusDeps.cmake

@@ -58,3 +58,31 @@ if(NOT TARGET absl::base)
   add_subdirectory(${CMAKE_BINARY_DIR}/abseil-src
                    ${CMAKE_BINARY_DIR}/abseil-build EXCLUDE_FROM_ALL)
 endif()
+
+if(NOT TARGET prometheus-cpp::core)
+  message(STATUS "Dependency: prometheus-cpp")
+
+  configure_file(${CMAKE_CURRENT_SOURCE_DIR}/cmake/prometheus-cpp.CMakeLists.txt
+                 ${CMAKE_BINARY_DIR}/prometheus-download/CMakeLists.txt)
+  execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
+                  RESULT_VARIABLE result
+                  WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/prometheus-download)
+  if(result)
+    message(FATAL_ERROR "CMake step failed: ${result}")
+  endif()
+  execute_process(COMMAND ${CMAKE_COMMAND} --build .
+                  RESULT_VARIABLE result
+                  WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/prometheus-download)
+  if(result)
+    message(FATAL_ERROR "Build step failed: ${result}")
+  endif()
+
+  set(ENABLE_PUSH OFF CACHE BOOL "Build prometheus-cpp push library" FORCE)
+  set(ENABLE_PULL OFF CACHE BOOL "Build prometheus-cpp pull library" FORCE)
+  set(ENABLE_COMPRESSION OFF
+      CACHE BOOL "Enable gzip compression for prometheus-cpp"
+      FORCE)
+  set(ENABLE_TESTING OFF CACHE BOOL "Build test for prometheus-cpp" FORCE)
+  add_subdirectory(${CMAKE_BINARY_DIR}/prometheus-src
+                   ${CMAKE_BINARY_DIR}/prometheus-build EXCLUDE_FROM_ALL)
+endif()

cmake/prometheus-cpp.CMakeLists.txt

@@ -0,0 +1,32 @@
+# Copyright 2018, OpenCensus Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+cmake_minimum_required(VERSION 3.5)
+
+project(prometheus-cpp-download NONE)
+
+include(ExternalProject)
+ExternalProject_Add(prometheus_cpp_project
+	GIT_REPOSITORY https://github.com/jupp0r/prometheus-cpp
+	GIT_TAG "master"
+	SOURCE_DIR "${CMAKE_BINARY_DIR}/prometheus-src"
+	BINARY_DIR "${CMAKE_BINARY_DIR}/prometheus-build"
+	UPDATE_COMMAND ""
+	PATCH_COMMAND ""
+	CONFIGURE_COMMAND ""
+	BUILD_COMMAND ""
+	INSTALL_COMMAND ""
+	TEST_COMMAND ""
+	LOG_DOWNLOAD ON
+)

opencensus/exporters/stats/CMakeLists.txt

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# add_subdirectory(prometheus) TODO
+add_subdirectory(prometheus)
 
 # add_subdirectory(stackdriver) TODO
 

opencensus/exporters/stats/prometheus/CMakeLists.txt

@@ -0,0 +1,36 @@
+# Copyright 2018, OpenCensus Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+opencensus_lib(exporters_stats_prometheus
+               PUBLIC
+               SRCS
+               internal/prometheus_exporter.cc
+               DEPS
+               exporters_stats_prometheus_utils
+               stats)
+
+opencensus_lib(exporters_stats_prometheus_utils
+               SRCS
+               internal/prometheus_utils.cc
+               DEPS
+               stats
+               absl::strings
+               absl::time
+               prometheus-cpp::core)
+
+opencensus_test(exporters_stats_prometheus_utils_test
+                internal/prometheus_utils_test.cc
+                exporters_stats_prometheus_utils
+                stats
+                stats_test_utils)