protobuf-maven-plugin does not generate java artifacts - maven-plugin

Question

I have below maven configuration and I am not able to figure out why java artifacts are not generated for my proto. My Proto file is in folder
/src/main/proto/hello.proto
I see no errors in my console when I do
mvn package
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.XXX.grpc</groupId>
<artifactId>grpc-poc-client</artifactId>
<version>1.0-SNAPSHOT</version>
<name>grpc-poc-client</name>
<!-- FIXME change it to the project's website -->
<url>http://www.example.com</url>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<maven.compiler.source>1.7</maven.compiler.source>
<maven.compiler.target>1.7</maven.compiler.target>
</properties>
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.11</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>io.grpc</groupId>
<artifactId>grpc-netty</artifactId>
<version>1.7.0</version>
</dependency>
<dependency>
<groupId>io.grpc</groupId>
<artifactId>grpc-protobuf</artifactId>
<version>1.7.0</version>
</dependency>
<dependency>
<groupId>io.grpc</groupId>
<artifactId>grpc-stub</artifactId>
<version>1.7.0</version>
</dependency>
</dependencies>
<build>
<extensions>
<extension>
<groupId>kr.motd.maven</groupId>
<artifactId>os-maven-plugin</artifactId>
<version>1.5.0.Final</version>
</extension>
</extensions>
<pluginManagement>
<plugins>
<plugin>
<groupId>org.xolstice.maven.plugins</groupId>
<artifactId>protobuf-maven-plugin</artifactId>
<version>0.5.0</version>
<configuration>
<clearOutputDirectory>false</clearOutputDirectory>
<protocArtifact>com.google.protobuf:protoc:3.4.0:exe:${os.detected.classifier}</protocArtifact>
<pluginId>grpc-java</pluginId>
<pluginArtifact>io.grpc:protoc-gen-grpc-java:1.7.0:exe:${os.detected.classifier}</pluginArtifact>
</configuration>
<executions>
<execution>
<goals>
<goal>compile</goal>
<goal>compile-custom</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</pluginManagement>
</build>
</project>
I am following the sample from
https://codelabs.developers.google.com/codelabs/cloud-grpc-java/index.html?index=..%2F..%2Findex#2