Fix GitLab Exporter default listenAddr

What does this MR do?

Fix GitLab Exporter default listenAddr

We should use `*` instead of `::` to make it compatible with IPv4 by
default.

Changelog: fixed

As part of !4072 (merged) we introduced IPv4 and IPv6 multiple compatibility. But Webrick services need to listen to * to be activated for both versions. If setting to :: it only binds to IPv6. We did this change to Sidkiq configs but forgot to do it for the GitLab Exporter.

Related issues

Closes gitlab-exporter(15.6.0) /metrics endpoint retur... (#6113 - closed).

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