fix(deps): update dependency prometheus-client to ^0.21.0 (#285)

This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
|
[prometheus-client](https://redirect.github.com/prometheus/client_python)
| `^0.20.0` -> `^0.21.0` |
[![age](https://developer.mend.io/api/mc/badges/age/pypi/prometheus-client/0.21.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![adoption](https://developer.mend.io/api/mc/badges/adoption/pypi/prometheus-client/0.21.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![passing](https://developer.mend.io/api/mc/badges/compatibility/pypi/prometheus-client/0.20.0/0.21.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![confidence](https://developer.mend.io/api/mc/badges/confidence/pypi/prometheus-client/0.20.0/0.21.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|

---

### Release Notes

<details>
<summary>prometheus/client_python (prometheus-client)</summary>

###
[`v0.21.0`](https://redirect.github.com/prometheus/client_python/releases/tag/v0.21.0):
0.21.0 / 2024-09-20

[Compare
Source](https://redirect.github.com/prometheus/client_python/compare/v0.20.0...v0.21.0)

#### What's Changed

\[CHANGE] Reject invalid (not GET or OPTION) HTTP methods.
[#&#8203;1019](https://redirect.github.com/prometheus/client_python/issues/1019)
\[ENHANCEMENT] Allow writing metrics when holding a lock for the metric
in the same thread.
[#&#8203;1014](https://redirect.github.com/prometheus/client_python/issues/1014)
\[BUGFIX] Check for and error on None label values.
[#&#8203;1012](https://redirect.github.com/prometheus/client_python/issues/1012)
\[BUGFIX] Fix timestamp comparison.
[#&#8203;1038](https://redirect.github.com/prometheus/client_python/issues/1038)

</details>

---

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---

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pyproject.toml

@@ -24,7 +24,7 @@ python = ">=3.9,<4.0"
 click = "^8.0.1"
 httpx = "^0.27.0"
 lxml = "^5.0.0"
-prometheus-client = "^0.20.0"
+prometheus-client = "^0.21.0"
 pydantic = "^2.0.0"
 typing-extensions = "^4.2.0"
 python-dotenv = "^1.0.0"