style: Add errorprone warnings. (#592)

* style: Add errorprone warnings.

Related to #590.

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: kolea2 <45548808+kolea2@users.noreply.github.com>

Author: 3 people

README.md

@@ -56,13 +56,13 @@ implementation 'com.google.cloud:google-cloud-datastore'
 If you are using Gradle without BOM, add this to your dependencies
 
 ```Groovy
-implementation 'com.google.cloud:google-cloud-datastore:2.2.6'
+implementation 'com.google.cloud:google-cloud-datastore:2.2.7'
 ```
 
 If you are using SBT, add this to your dependencies
 
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-datastore" % "2.2.6"
+libraryDependencies += "com.google.cloud" % "google-cloud-datastore" % "2.2.7"
 ```
 
 ## Authentication

pom.xml

@@ -143,6 +143,7 @@
     <github.global.server>github</github.global.server>
     <site.installationModule>google-cloud-datastore-parent</site.installationModule>
     <project.javadoc.protobufBaseURL>https://googleapis.dev/java/google-api-grpc/latest</project.javadoc.protobufBaseURL>
+    <error-prone.version>2.10.0</error-prone.version>
   </properties>
 
   <dependencyManagement>
@@ -210,6 +211,49 @@
     </pluginManagement>
   </build>
 
+  <profiles>
+    <profile>
+      <id>java9</id>
+      <activation>
+        <jdk>[9,)</jdk>
+      </activation>
+      <build>
+        <plugins>
+          <plugin>
+            <groupId>org.apache.maven.plugins</groupId>
+            <artifactId>maven-compiler-plugin</artifactId>
+            <version>3.8.0</version>
+            <configuration>
+              <encoding>UTF-8</encoding>
+              <fork>true</fork>
+              <compilerArgs>
+                <arg>-XDcompilePolicy=simple</arg>
+                <arg>-Xplugin:ErrorProne</arg>
+                <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED</arg>
+                <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED</arg>
+                <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED</arg>
+                <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.model=ALL-UNNAMED</arg>
+                <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED</arg>
+                <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED</arg>
+                <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED</arg>
+                <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED</arg>
+                <arg>-J--add-opens=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED</arg>
+                <arg>-J--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED</arg>
+              </compilerArgs>
+              <annotationProcessorPaths>
+                <path>
+                  <groupId>com.google.errorprone</groupId>
+                  <artifactId>error_prone_core</artifactId>
+                  <version>${error-prone.version}</version>
+                </path>
+              </annotationProcessorPaths>
+            </configuration>
+          </plugin>
+        </plugins>
+      </build>
+    </profile>
+  </profiles>
+
   <modules>
     <module>google-cloud-datastore</module>
     <module>proto-google-cloud-datastore-v1</module>