chore: regenerate README (#632)

This PR was generated using Autosynth. 🌈


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

```
2021-08-10 02:05:16,686 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-monitoring/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-08-10 02:05:17,892 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/3954f283-cf9c-43a5-9d25-de7883551710/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": "398ddc9d7b38651221f66b98b86a5cebe97d3168"
+        "sha": "de80e1852d9f9bb8fbd9bc776c9c122dd25cae3d"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "3d32990b6f4b57be77455e3ca89e6e21cd0c06b3"
+        "sha": "6252f2cd074c38f37b44abe5e96d128733eb1b61"
       }
     }
   ]

README.md

@@ -42,7 +42,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-monitoring</artifactId>
-  <version>2.3.5-SNAPSHOT</version>
+  <version>3.0.0</version>
 </dependency>
 
 ```