[BUG] GitHub App auth on GHES still uses api.github.com for installation token refresh instead of the configured API_URL

[Ahmadshata]

Summary

When github-exporter is configured to use GitHub Enterprise Server (GHES) with GitHub App authentication, the exporter still tries to fetch the installation access token from https://api.github.com/app/installations/.../access_tokens instead of the configured GHES API URL.

This causes GitHub App auth to fail on GHES with 401 Unauthorized.

It looks like PR #131 fixed the regular client base URL handling, but the GitHub App installation transport still defaults to api.github.com.

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Error

time="2026-03-19T03:57:50Z" level=error msg=" getting rate limit metrics: fetching rate limits: Get \"https://{GHES_DOMAIN}/api/v3/rate_limit?per_page=100\": could not refresh installation id 580's token: received non 2xx response status \"401 Unauthorized\" when fetching https://api.github.com/app/installations/580/access_tokens"

As shown above:

• The exporter correctly calls the GHES rate limit endpoint:

https://{GHES_DOMAIN}/api/v3/rate_limit

But the GitHub App installation token refresh is still sent to:

https://api.github.com/app/installations/{INSTALLATION_ID}/access_tokens

Expected behavior

When API_URL is set to a GHES API endpoint, for example:

https://{GHES_DOMAIN}/api/v3

Then all GitHub App authentication calls should also use that same GHES base URL, including:

POST /app/installations/{INSTALLATION_ID}/access_tokens

So in this case, the request should go to:

https://{GHES_DOMAIN}/api/v3/app/installations/{INSTALLATION_ID}/access_tokens.

Why this matters

PR #131 fixed GHES support for the normal API client, but GHES + GitHub App auth still does not work because the installation token refresh path is still pointed at public GitHub.

So this seems to affect specifically:

• API_URL set to GHES

• GITHUB_APP=true

Environment

• github-exporter version: v.2.3.0

• Auth mode: GitHub App