Add Linkage Monitor presubmit check (#719)

* Linkage Monitor

* 2019

* empty commit to trigger Kokoro

* 755 permission

.kokoro/linkage-monitor.sh

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+#!/bin/bash
+# Copyright 2019 Google Inc.
+#
+# 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.
+
+set -eo pipefail
+
+cd github/google-http-java-client/
+
+# Print out Java
+java -version
+echo $JOB_TYPE
+
+export MAVEN_OPTS="-Xmx1024m -XX:MaxPermSize=128m"
+
+# Installs snapshot artifacts locally. Linkage Monitor uses them.
+mvn install -DskipTests=true -B -V
+
+# Kokoro job cloud-opensource-java/ubuntu/linkage-monitor-gcs creates this JAR
+JAR=linkage-monitor-latest-all-deps.jar
+curl -v -O "https://storage.googleapis.com/cloud-opensource-java-linkage-monitor/${JAR}"
+
+# Fails if there's new linkage errors compared with baseline
+java -jar $JAR com.google.cloud:libraries-bom

.kokoro/presubmit/linkage-monitor.cfg

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+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Linkage Monitor notifies new linkage errors as Kokoro presubmit check
+# https://github.com/GoogleCloudPlatform/cloud-opensource-java/tree/master/linkage-monitor
+
+# Configure the docker image for kokoro-trampoline.
+env_vars: {
+  key: "TRAMPOLINE_IMAGE"
+  value: "gcr.io/cloud-devrel-kokoro-resources/java8"
+}
+
+env_vars: {
+  key: "TRAMPOLINE_BUILD_FILE"
+  value: "github/google-http-java-client/.kokoro/linkage-monitor.sh"
+}