chore(deps): update dependency google-cloud-monitoring to v2.6.0 (GoogleCloudPlatform#6809)

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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) | `==2.5.1` -> `==2.6.0` | [![age](https://badges.renovateapi.com/packages/pypi/google-cloud-monitoring/2.6.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/google-cloud-monitoring/2.6.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/google-cloud-monitoring/2.6.0/compatibility-slim/2.5.1)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/google-cloud-monitoring/2.6.0/confidence-slim/2.5.1)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v2.6.0`](https://togithub.com/googleapis/python-monitoring/blob/master/CHANGELOG.md#&#8203;260-httpswwwgithubcomgoogleapispython-monitoringcomparev252v260-2021-10-07)

[Compare Source](https://togithub.com/googleapis/python-monitoring/compare/v2.5.1...v2.6.0)

##### Features

-   add context manager support in client ([#&GoogleCloudPlatform#8203;230](https://www.togithub.com/googleapis/python-monitoring/issues/230)) ([954dd18](https://www.github.com/googleapis/python-monitoring/commit/954dd18966520dbc623470ef528166d83d3e19ba))

##### [2.5.2](https://www.github.com/googleapis/python-monitoring/compare/v2.5.1...v2.5.2) (2021-09-30)

##### Bug Fixes

-   improper types in pagers generation ([4a8b823](https://www.github.com/googleapis/python-monitoring/commit/4a8b823061a03fd0e5c385c1fa0ac964b9a5597a))

##### [2.5.1](https://www.github.com/googleapis/python-monitoring/compare/v2.5.0...v2.5.1) (2021-09-24)

##### Bug Fixes

-   add 'dict' annotation type to 'request' ([ffae24c](https://www.github.com/googleapis/python-monitoring/commit/ffae24c816a3e1645464098cf86fdaf54a3064a9))

</details>

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trace/cloud-trace-demo-app-opentelemetry/app/requirements.txt

@@ -5,5 +5,5 @@ opentelemetry-propagator-gcp==1.0.0
 opentelemetry-instrumentation-flask==0.20b0
 grpcio==1.41.0
 httpretty==1.1.4
-google-cloud-monitoring==2.5.1
+google-cloud-monitoring==2.6.0
 google-cloud-trace==0.24.0

trace/cloud-trace-demo-app/app/requirements.txt

@@ -1,4 +1,4 @@
-google-cloud-monitoring==2.5.1
+google-cloud-monitoring==2.6.0
 google-cloud-trace==0.24.0
 opencensus==0.8.0
 # Don't upgrade to 2.0.0 yet until opencensus-ext-flask is okay with it