Add prometheous endpoints for reporting metrics [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Adds an endpoint that can by used by frontend to observe Prometheus metrics. The available metrics for this operation are:

• pipeline_graph_link_calculation_duration_seconds
• pipeline_graph_links_total
• pipeline_graph_link_per_job_ratio

Related to #281351 (closed)

curl \
  -X POST \
  -H "Content-Type: application/json" \
  http://gdev.bobin.ro:3000/root/fallback-cache/-/ci/prometheus_metrics/histograms.json \
    --data '{"histograms": [{"name": "pipeline_graph_link_calculation_duration_seconds", "value": "5"}]}'

For this curl command to work, an authenticity token is required in the request headers or skip_before_action :verify_authenticity_token should be added in the controller.

Screenshots (strongly suggested)

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