chore: change [START_EXCLUDE] annotation to [START_EXCLUDE silent]

The `[START_EXCLUDE]` annotation lets us omit part of Cloud Monitoring's pom.xml file from the documentation on cloud.google.com. However, the text before `[START_EXCLUDE]` isn't stripped out by default. As a result, the docs end up with something like this:

```xml
<dependencies>
  <dependency>
    <groupId>com.google.cloud</groupId>
    <artifactId>google-cloud-monitoring</artifactId>
  </dependency>
  <!-- ...
</dependencies>
```

By changing the annotation to `[START_EXCLUDE silent]`, we prevent the line `<!-- ...` from appearing in the docs.

Author: hegemonic

monitoring/v3/pom.xml

@@ -19,7 +19,7 @@
   <groupId>com.example.monitoring</groupId>
   <artifactId>monitoring-google-cloud-v3-samples</artifactId>
   <version>0.1-SNAPSHOT</version>
-  <packaging>jar</packaging>  
+  <packaging>jar</packaging>
   <!--
     The parent pom defines common style checks and testing strategies for our samples.
     Removing or replacing it should not affect the execution of the samples in anyway.
@@ -28,11 +28,11 @@
     <groupId>com.google.cloud.samples</groupId>
     <artifactId>shared-configuration</artifactId>
     <version>1.2.0</version>
-  </parent>  
+  </parent>
   <properties>
     <maven.compiler.target>1.8</maven.compiler.target>
     <maven.compiler.source>1.8</maven.compiler.source>
-  </properties>  
+  </properties>
   <!-- [START monitoring_install_with_bom] -->
   <dependencyManagement>
     <dependencies>
@@ -44,13 +44,13 @@
         <scope>import</scope>
       </dependency>
     </dependencies>
-  </dependencyManagement>  
+  </dependencyManagement>
   <dependencies>
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-monitoring</artifactId>
     </dependency>
-    <!-- [START_EXCLUDE] -->
+    <!-- [START_EXCLUDE silent] -->
     <dependency>
       <groupId>commons-cli</groupId>
       <artifactId>commons-cli</artifactId>