chore: regenerate README (#569)

This PR was generated using Autosynth. 🌈


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

```
2021-05-31 22:30:34,726 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-monitoring/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-05-31 22:30:36,425 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/ff524262-c034-4487-bac0-fcff0b21efa0/targets

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

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-monitoring.git",
-        "sha": "d17d470f2aeda7b0d7a30ac3ccdc94d183f822e9"
+        "sha": "f9ba30d2e1da96953333283b2ef284a1d4676859"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "a2de127f90799cd5bace653b1dcc5dd2ca8b19c9"
+        "sha": "c3e41da0fa256ad7f6b4bc76b9d069dedecdfef4"
       }
     }
   ]

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>2.2.4-SNAPSHOT</version>
+  <version>2.3.0</version>
 </dependency>
 
 ```