Bump protobuf in /docs/examples/cloud_trace_exporter (#200)

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.15.8 to 3.18.3.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.15.8...v3.18.3)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-type: direct:production
...

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docs/examples/cloud_trace_exporter/requirements.txt

@@ -12,7 +12,7 @@ opentelemetry-api==1.0.0
 ../../../opentelemetry-exporter-gcp-trace/
 opentelemetry-sdk==1.0.0
 packaging==20.9
-protobuf==3.15.8
+protobuf==3.18.3
 pyasn1==0.4.8
 pyasn1-modules==0.2.8
 pyparsing==2.4.7