chore: regenerate README (#555)

This PR was generated using Autosynth. 🌈


<details><summary>Log from Synthtool</summary>

```
2021-05-17 01:53:56,683 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-monitoring/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-05-17 01:53:58,204 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

Full log will be available here:
https://source.cloud.google.com/results/invocations/410df9cb-d2c1-45d4-bf2b-d9220aea766b/targets

- [ ] To automatically regenerate this PR, check this box. (May take up to 24 hours.)

.github/readme/synth.metadata/synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-monitoring.git",
-        "sha": "afeee4407903f62978dc6f015542646975b2c4f2"
+        "sha": "ba5a44e6c98367c236143131518eed5969e9bca1"
       }
     },
     {

README.md

@@ -17,7 +17,7 @@ If you are using Maven with [BOM][libraries-bom], add this to your pom.xml file
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>libraries-bom</artifactId>
-      <version>20.3.0</version>
+      <version>20.4.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>
@@ -46,18 +46,18 @@ If you are using Maven without BOM, add this to your dependencies:
 
 If you are using Gradle 5.x or later, add this to your dependencies
 ```Groovy
-implementation platform('com.google.cloud:libraries-bom:20.3.0')
+implementation platform('com.google.cloud:libraries-bom:20.4.0')
 
 compile 'com.google.cloud:google-cloud-monitoring'
 ```
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-monitoring:2.2.2'
+compile 'com.google.cloud:google-cloud-monitoring:2.2.3'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-monitoring" % "2.2.2"
+libraryDependencies += "com.google.cloud" % "google-cloud-monitoring" % "2.2.3"
 ```
 
 ## Authentication