Update prometheus-client requirement from >=0.19.0 to >=0.25.0 in /src

Updates the requirements on [prometheus-client](https://github.com/prometheus/client_python) to permit the latest version.
- [Release notes](https://github.com/prometheus/client_python/releases)
- [Commits](prometheus/client_python@v0.19.0...v0.25.0)

---
updated-dependencies:
- dependency-name: prometheus-client
  dependency-version: 0.25.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

src/a2a/requirements_a2a.txt

@@ -1,10 +1,10 @@
-fastapi>=0.104.0
+fastapi>=0.104.0
 uvicorn[standard]>=0.24.0
 starlette>=0.27.0
 pydantic>=2.5.0
 aiofiles>=23.2.0
 httpx>=0.25.0
 psutil>=5.9.0
-prometheus-client>=0.19.0
+prometheus-client>=0.25.0
 gunicorn>=21.2.0
 aiosignal>=1.3.0