Avoid warnings from Prometheus client in development/test Rails environment

What does this MR do?

Avoid warnings from Prometheus client in dev/test

Set ennvar prometheus_multiproc_dir before initializing Prometheus client's configuration.o

After https://gitlab.com/gitlab-org/prometheus-client-mmap/commit/7a8bb0f Prometheus client will warn if the envvar prometheus_multiproc_dir is not set.

See also https://gitlab.com/gitlab-org/prometheus-client-mmap/issues/29

The warning on GDK

@tkuah on Slack:

I'm seeing above now on rails db:migrate and also on many spec runs. Is it an issue ?

W, [2019-07-01T12:28:40.619652 #46649]  WARN -- : Please set prometheus_multiproc_dir to persistent directory path. Temporarily writing mmap files to /var/folders/_6/dnrbb2cj01l2_9k407091t4c0000gn/T/prometheus-mmap20190701-46649-17zvpsu

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
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Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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