@LenaHoinkis Thank you for this PR -- the main idea to support "EnterpriseClient" is welcome. Thank you for preserving the unit test coverage!

I've added a few suggestions for how organize the feature in a way that I think will make the change easier for us to maintain going forward.

Reviewable status: 0 of 1 LGTMs obtained

cmd/github_receiver/main.go, line 45 at r2 (raw file):

	githubOrg       = flag.String("org", "", "The github user or organization name where all repos are found.")
	githubRepo      = flag.String("repo", "", "The default repository for creating issues when alerts do not include a repo label.")
	githubBaseURL   = flag.String("baseURL", "", "The URL of your GitHub Enterprise with API suffix (for example '/api/v3/').")

Please name these flags like:

• enterprise.base-url
• enterprise.upload-url

issues/issues.go, line 105 at r2 (raw file):

			uploadURL = baseURL
		}
		githubClient, err = github.NewEnterpriseClient(baseURL, uploadURL, oauth2.NewClient(ctx, tokenSource))

Hmm, it looks like the EnterpriseClient behaves very differently from the default client (it enforces certain api version paths).

In that case, I would rather see two functions, NewClient (as before), and NewEnterpriseClient. Then main can select which to call based on the given flags. There is already some client-type selection logic in main, this would be a third choice.

@LenaHoinkis Great. One more small formatting, and I'll merge the change, and tag a new release.

Reviewable status: 0 of 1 LGTMs obtained

cmd/github_receiver/main.go, line 129 at r3 (raw file):

	if *enableInMemory {
		client = local.NewClient()
	} else {

Please structure at the same level, like:

if ... {
    // first condition
} else if {
    // second condition
} else {
    // third condition
}

Reviewable status: complete! 1 of 1 LGTMs obtained