Remove autocomplete for prometheus manual configs

What does this MR do?

The current Prometheus manual configuration form has inputs that can have moderately sensitive information like the client_id for a Prometheus/Thanos instance that is behind an authentication proxy. This MR sets autocomplete to off for such inputs.

While client_id does not contain highly sensitive information, in my opinion, to be safe we're still doing it for the time being. This can be easily removed if appsec team deems it as unnecessary. We're waiting for appsec to respond here #216859 (comment 355575534)

Note

This is behind prometheus_service_iap_auth feature flag.

Screenshots

before	after

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• 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