chore: regenerate README (#486)

This PR was generated using Autosynth. 🌈


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

```
2021-04-12 23:02:31,897 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-monitoring/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-04-12 23:02:33,149 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/9d2d185d-ed1e-4b12-ba8d-1ab892bd5550/targets

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-monitoring.git",
-        "sha": "f23a57b4ad69fbd42e07a4ed98c6e30ce37b52ac"
+        "sha": "f047185838aa5fb4a80e43354f0d34b313821339"
       }
     },
     {

README.md

@@ -39,7 +39,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-monitoring</artifactId>
-  <version>2.1.1-SNAPSHOT</version>
+  <version>2.2.0</version>
 </dependency>
 
 ```