_setTestMetadata(metadata, env) {
    for (const [key, envVar] of Object.entries(GRPC_TEST_META)) {
      const value = parseInt(env[envVar], 10)
      if (value) {
        logger.trace('Adding %s metadata: %s', key, value)
        metadata.add(key, value)
      }
    }
  }
Adds test metadata used to simulate connectivity issues
when appropriate env vars are set

@param {object} metadata metadata to set
@param {object} env process.env