Add Prometheus series API to allowed list

What does this MR do?

Allow the Prometheus series API to be proxied by the Prometheus::ProxyService.

The series API will be called by the metrics dashboard page on the frontend, in order to populate variable dropdowns.

This is one of the MRs needed to implement support for variables whose possible values are all the different values of a Prometheus label. This variable will be similar to Grafana's label_values(metric, label).

Issue: #214539 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry ~backstage
• [-] Documentation (if required) Internal API
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] 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
• [-] Security reports checked/validated by a reviewer from the AppSec team