Revert gitlab-exporter default server to Puma

What does this MR do?

We recently made a change that defaults the gitlab-exporter Rack server to webrick instead of Puma, which was misguided. We found that the excess memory use was caused by other sources. Since Puma is easier to scale out due it being multi-threaded, all else being equal, it is the better choice after all.

We would like to retain the overall ability to swap out the Rack server, however, so we're only changing the default value.

See also gitlab-org/omnibus-gitlab!4922 (merged) in which we propose changes that improve memory use regardless of which server is used.

Related issues

• gitlab-org/gitlab#297241 (closed)
• gitlab-org&5035

Omnibus MR: gitlab-org/omnibus-gitlab!4946 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes: See gitlab-org/omnibus-gitlab!4896 (merged)
• [-] Documentation created/updated
• [-] Tests added
• [-] Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened