Add reviewer to external PRs (#426)

Author: trask

.github/workflows/reusable-create-collector-contrib-pull-request.yml

@@ -72,5 +72,6 @@ jobs:
           git push --set-upstream origin HEAD:update-opentelemetry-jmx-metrics-to-${VERSION}
           gh pr create --title "$message" \
                        --body "$body" \
+                       --reviewer open-telemetry/java-instrumentation-approvers \
                        --repo open-telemetry/opentelemetry-collector-contrib \
                        --base main