ci: setting up linkage monitor Kokoro build config for gax-java

.kokoro/linkage-monitor.sh

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+#!/bin/bash
+# Copyright 2020 Google LLC
+#
+# 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
+# Display commands being run.
+set -x
+
+cd github/gax-java/
+
+# Print out Java version
+java -version
+echo ${JOB_TYPE}
+
+# Using Gradle's maven-publish plugin. No need to sign the JAR.
+./gradlew build publishToMavenLocal -x test -x signMavenJavaPublication
+
+# Kokoro job cloud-opensource-java/ubuntu/linkage-monitor-gcs creates this JAR
+JAR=linkage-monitor-latest-all-deps.jar
+curl -v -O "https://storage.googleapis.com/cloud-opensource-java-linkage-monitor/${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
+
+# 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/gax-java/.kokoro/linkage-monitor.sh"
+}