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

GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples