chore(deps): update dependency google-cloud-monitoring to v2.4.0 (#172)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [google-cloud-monitoring](https://togithub.com/googleapis/python-monitoring) | `==2.3.0` -> `==2.4.0` | [![age](https://badges.renovateapi.com/packages/pypi/google-cloud-monitoring/2.4.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/google-cloud-monitoring/2.4.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/google-cloud-monitoring/2.4.0/compatibility-slim/2.3.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/google-cloud-monitoring/2.4.0/confidence-slim/2.3.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>googleapis/python-monitoring</summary>

### [`v2.4.0`](https://togithub.com/googleapis/python-monitoring/blob/master/CHANGELOG.md#&#8203;240-httpswwwgithubcomgoogleapispython-monitoringcomparev230v240-2021-07-01)

[Compare Source](https://togithub.com/googleapis/python-monitoring/compare/v2.3.0...v2.4.0)

##### Features

-   add always_use_jwt_access ([#&#8203;165](https://www.github.com/googleapis/python-monitoring/issues/165)) ([064f1e0](https://www.github.com/googleapis/python-monitoring/commit/064f1e0a8df02c04bdae6b13c645f2b399c2c1ef))

##### Bug Fixes

-   disable always_use_jwt_access ([#&#8203;171](https://www.github.com/googleapis/python-monitoring/issues/171)) ([c194a00](https://www.github.com/googleapis/python-monitoring/commit/c194a00031763153bcc67346328a02b85fabc359))

##### Documentation

-   omit mention of Python 2.7 in 'CONTRIBUTING.rst' ([#&#8203;1127](https://www.github.com/googleapis/python-monitoring/issues/1127)) ([#&#8203;159](https://www.github.com/googleapis/python-monitoring/issues/159)) ([adc82c9](https://www.github.com/googleapis/python-monitoring/commit/adc82c9d1812fb0efe00cbfa09f19e8c04277881)), closes [#&#8203;1126](https://www.github.com/googleapis/python-monitoring/issues/1126)

</details>

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samples/snippets/v3/alerts-client/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-monitoring==2.3.0
+google-cloud-monitoring==2.4.0
 tabulate==0.8.9

samples/snippets/v3/cloud-client/requirements.txt

@@ -1 +1 @@
-google-cloud-monitoring==2.3.0
+google-cloud-monitoring==2.4.0

samples/snippets/v3/uptime-check-client/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-monitoring==2.3.0
+google-cloud-monitoring==2.4.0
 tabulate==0.8.9