deps: update dependency com.google.api.grpc:proto-google-cloud-monitoring-v3 to v3.56.0 (#3563)

This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.api.grpc:proto-google-cloud-monitoring-v3](https://redirect.github.com/googleapis/google-cloud-java) | `3.55.0` -> `3.56.0` | [![age](https://developer.mend.io/api/mc/badges/age/maven/com.google.api.grpc:proto-google-cloud-monitoring-v3/3.56.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://developer.mend.io/api/mc/badges/adoption/maven/com.google.api.grpc:proto-google-cloud-monitoring-v3/3.56.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://developer.mend.io/api/mc/badges/compatibility/maven/com.google.api.grpc:proto-google-cloud-monitoring-v3/3.55.0/3.56.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://developer.mend.io/api/mc/badges/confidence/maven/com.google.api.grpc:proto-google-cloud-monitoring-v3/3.55.0/3.56.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) |

---

### Configuration

📅 **Schedule**: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

---

 - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check this box

---

This PR was generated by [Mend Renovate](https://mend.io/renovate/). View the [repository job log](https://developer.mend.io/github/googleapis/java-spanner).
<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiIzOS42OS4zIiwidXBkYXRlZEluVmVyIjoiMzkuOTIuMCIsInRhcmdldEJyYW5jaCI6Im1haW4iLCJsYWJlbHMiOltdfQ==-->

Author: renovate-bot

google-cloud-spanner/pom.xml

@@ -270,7 +270,7 @@
     <dependency>
       <groupId>com.google.api.grpc</groupId>
       <artifactId>proto-google-cloud-monitoring-v3</artifactId>
-      <version>3.55.0</version>
+      <version>3.56.0</version>
     </dependency>
     <dependency>
       <groupId>com.google.auth</groupId>