Authentication fails on GitHub Enterprise Server

[shanemalachow-SL]

Looks like there's not proper handling of instances of GitHub Enterprise Server with regards to authentication and queries against the API. When authenticating to GitHub Enterprise Server installations with github3 you need to specifically use the GitHubEnterprise object and pass in the URL of the server, and then use that for the rest of the calls. Looks like the code in

issue-metrics/issue_metrics.py

Line 116 in 92b849a

def auth_to_github() -> github3.GitHub:

only really uses the default object, which will interact with the GitHub.com API. The environment variables should be able to be used to determine whether this is a GitHub.com workflow or an enterprise server workflow, specifically GITHUB_SERVER_URL which is already passed into the Docker environment for the action.