testing/opa: reenable
https://github.com/open-policy-agent/opa/pull/6218 PR added the missing metric for Prometheus test, which was added in Go 1.21.1, hence fixing the build.
https://github.com/open-policy-agent/opa/pull/6218 PR added the missing metric for Prometheus test, which was added in Go 1.21.1, hence fixing the build.