Resolve "Use Gitlab::HTTP in PrometheusClient instead of RestClient"

What does this MR do?

Change use of RestClient to Gitlab::HTTP in Prometheus Client

To standardize on one HTTP client, change RestClient to Gitlab::HTTP for PrometheusClient

• Change PrometheusClient#new to accept a URL and options instead of a RestClient::Resource object
• Adapt PrometheusAdapter, PrometheusService to the new initializer
• Rescue Gitlab::HTTP exceptions instead of RestClient exceptions
• Translate some RestClient options (:ssl_cert_store into :cert_store) sent by Gitlab::Kubernetes::KubeClient (aka kubeclient gem) into Gitlab::HTTP compatible options.

Risks:

• Destabilization of the Prometheus interface (pulling stats from Prometheus)

Mitigations:

• Additional test coverage to address previously untested code paths.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

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

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

Closes #60024 (closed)