addLabel = async (octokit, prNumber, owner, repo, labels) => {
  await octokit.rest.issues.addLabels({
    owner,
    repo,
    issue_number: prNumber,
    labels,
  });
}
Add label to pull request.

@param {Object} octokit Octokit instance.
@param {number} prNumber Pull request number.
@param {string} owner Repository owner.
@param {string} repo Repository name.
@param {string[]} labels Labels to add.
@returns {Promise<void>} Promise.