Bump prometheus-client from 0.20.0 to 0.21.0 (#17746)

Bumps [prometheus-client](https://github.com/prometheus/client_python)
from 0.20.0 to 0.21.0.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a
href="https://github.com/prometheus/client_python/releases">prometheus-client's
releases</a>.</em></p>
<blockquote>
<h2>0.21.0 / 2024-09-20</h2>
<h2>What's Changed</h2>
<p>[CHANGE] Reject invalid (not GET or OPTION) HTTP methods. <a
href="https://redirect.github.com/prometheus/client_python/issues/1019">#1019</a>
[ENHANCEMENT] Allow writing metrics when holding a lock for the metric
in the same thread. <a
href="https://redirect.github.com/prometheus/client_python/issues/1014">#1014</a>
[BUGFIX] Check for and error on None label values. <a
href="https://redirect.github.com/prometheus/client_python/issues/1012">#1012</a>
[BUGFIX] Fix timestamp comparison. <a
href="https://redirect.github.com/prometheus/client_python/issues/1038">#1038</a></p>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/prometheus/client_python/commit/3b183b44994454be226c208037e1fe4b9a89dfc5"><code>3b183b4</code></a>
Release 0.21.0</li>
<li><a
href="https://github.com/prometheus/client_python/commit/0014e9776350a252930671ed170edee464f9b428"><code>0014e97</code></a>
Use re-entrant lock. (<a
href="https://redirect.github.com/prometheus/client_python/issues/1014">#1014</a>)</li>
<li><a
href="https://github.com/prometheus/client_python/commit/7c45f84e5e3d2e0a75b3946408fec1a4d5c72841"><code>7c45f84</code></a>
Reject invalid HTTP methods and resources (<a
href="https://redirect.github.com/prometheus/client_python/issues/1019">#1019</a>)</li>
<li><a
href="https://github.com/prometheus/client_python/commit/09a5ae30602a7a81f6174dae4ba08b93ee7feed2"><code>09a5ae3</code></a>
Fix timestamp comparison (<a
href="https://redirect.github.com/prometheus/client_python/issues/1038">#1038</a>)</li>
<li><a
href="https://github.com/prometheus/client_python/commit/e364a96f506bbb70ae744e0b3307e4b693e28258"><code>e364a96</code></a>
Fix a typo in ASGI docs (<a
href="https://redirect.github.com/prometheus/client_python/issues/1036">#1036</a>)</li>
<li><a
href="https://github.com/prometheus/client_python/commit/eeec421b2f489d2c465bb8ca419b772829b7b16c"><code>eeec421</code></a>
Pin python 3.8 and 3.9 at patch level (<a
href="https://redirect.github.com/prometheus/client_python/issues/1024">#1024</a>)</li>
<li><a
href="https://github.com/prometheus/client_python/commit/7bc8cddfbbc9b72c98725a879d9b94a675a6c7da"><code>7bc8cdd</code></a>
docs: correct link to multiprocessing docs (<a
href="https://redirect.github.com/prometheus/client_python/issues/1023">#1023</a>)</li>
<li><a
href="https://github.com/prometheus/client_python/commit/4535ce0f43097aa48e44a65747d82064f2aadaf5"><code>4535ce0</code></a>
Add sanity check for label value (<a
href="https://redirect.github.com/prometheus/client_python/issues/1012">#1012</a>)</li>
<li>See full diff in <a
href="https://github.com/prometheus/client_python/compare/v0.20.0...v0.21.0">compare
view</a></li>
</ul>
</details>
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