Bump protobuf in /docs/examples/prometheus_exemplars

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 4.25.3 to 4.25.8.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/protobuf_release.bzl)
- [Commits](protocolbuffers/protobuf@v4.25.3...v4.25.8)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-version: 4.25.8
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

docs/examples/prometheus_exemplars/requirements.txt

@@ -72,7 +72,7 @@ packaging==24.1
     # via opentelemetry-instrumentation-flask
 prometheus-client==0.20.0
     # via -r requirements.in
-protobuf==4.25.3
+protobuf==4.25.8
     # via
     #   googleapis-common-protos
     #   opentelemetry-proto