POC API for Prometheus metrics

What does this MR do?

Incrementing a metric for the project with id 1(this project needs to be public):

curl -X POST -H "Content-Type: application/json" http://localhost:3000/api/v4/projects/1/prometheus_metrics/draw_links --data '{"duration": "10"}'

Going to localhost:3000/-/metrics shows:

# HELP draw_links_total_duration_seconds Multiprocess metric
# TYPE draw_links_total_duration_seconds histogram
draw_links_total_duration_seconds_bucket{project="root/cancel-pipeline",le="+Inf"} 2
draw_links_total_duration_seconds_bucket{project="root/cancel-pipeline",le="0.005"} 0
draw_links_total_duration_seconds_bucket{project="root/cancel-pipeline",le="0.01"} 0
draw_links_total_duration_seconds_bucket{project="root/cancel-pipeline",le="0.025"} 0
draw_links_total_duration_seconds_bucket{project="root/cancel-pipeline",le="0.05"} 0
draw_links_total_duration_seconds_bucket{project="root/cancel-pipeline",le="0.1"} 0
draw_links_total_duration_seconds_bucket{project="root/cancel-pipeline",le="0.25"} 0
draw_links_total_duration_seconds_bucket{project="root/cancel-pipeline",le="0.5"} 0
draw_links_total_duration_seconds_bucket{project="root/cancel-pipeline",le="1"} 0
draw_links_total_duration_seconds_bucket{project="root/cancel-pipeline",le="10"} 2
draw_links_total_duration_seconds_bucket{project="root/cancel-pipeline",le="2.5"} 0
draw_links_total_duration_seconds_bucket{project="root/cancel-pipeline",le="5"} 0
draw_links_total_duration_seconds_count{project="root/cancel-pipeline"} 2
draw_links_total_duration_seconds_sum{project="root/cancel-pipeline"} 20

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