Add GraphQL interface for linked metrics for issues

What does this MR do and why?

• Adds a new column project_id to observability_metrics_issues_connections to more easily query a project's metrics and connected issues.
  • Adds matching foreign key, and indexes as required.
• Adds a GraphQL interface to query those connections.
  • This requires new types, enums, resolvers and associations on the appropriate models.
• Adds tests for all new files and updated code.

Relates to gitlab-org/opstrace/opstrace#2903 (closed)

Query plans

All metrics for a project

SELECT "observability_metrics_issues_connections".*
FROM "observability_metrics_issues_connections"
WHERE "observability_metrics_issues_connections"."project_id" = 7

                                       QUERY PLAN
-----------------------------------------------------------------------------------------
 Seq Scan on observability_metrics_issues_connections  (cost=0.00..1.09 rows=1 width=82)
   Filter: (project_id = 7)
(2 rows)

Filtered metric for a project

SELECT "observability_metrics_issues_connections".*
FROM "observability_metrics_issues_connections"
WHERE "observability_metrics_issues_connections"."project_id" = 7
AND "observability_metrics_issues_connections"."metric_name" = 'te343st2345'
AND "observability_metrics_issues_connections"."metric_type" = 1

                                         QUERY PLAN
--------------------------------------------------------------------------------------------
 Seq Scan on observability_metrics_issues_connections  (cost=0.00..1.12 rows=1 width=82)
   Filter: ((project_id = 7) AND (metric_name = 'te343st2345'::text) AND (metric_type = 1))
(2 rows)

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Before	After

How to set up and validate locally

1. Ensure you're using an Ultimate namespace.

2. Enable the :observability_features feature flag.

3. Create a new project, within an Ultimate group.

4. Create two new issues, and note their ID.

5. On a rails console, create a few linked metrics for that project. Repeat this for each issue.

   issue_id = {{ID}}
   a = Observability::MetricsIssuesConnection.new(metric_type: :sum_type, metric_name: "test2345", issue: Issue.find(issue_id))
   a.save! # Raise an exception if something went wrong.

6. This will create two metric links for the same metric (denoted by metric_type, and metric_name) in the same project, across two different issues.

7. For the created metric, we can query a list of links: (type is a newly defined GQL enum of OpenTelemetry metric types)

   query {
     project(fullPath: "Flightjs/Flight") {
       id
       observabilityMetricsLinks(name: "test2345", type: SUM_TYPE) {
         nodes {
           name
           type
           issue {
             title
             webUrl
           }
         }
       }
     }
   }

8. If successful, it should return a list of observabiltyMetrics, each representing the link between an observabililtyMetric and its issue (if queried, as above)

   {
     "data": {
       "project": {
         "id": "gid://gitlab/Project/7",
         "observabilityMetricsLinks": {
           "nodes": [
             {
               "name": "test2345",
               "type": "sum_type",
               "issue": {
                 "title": "fdfsdfsdfsdfsdfs",
                 "webUrl": "http://127.0.0.1:3000/flightjs/Flight/-/issues/40"
               }
             },
             {
               "name": "test2345",
               "type": "sum_type",
               "issue": {
                 "title": "gdfgdfgdfgdf",
                 "webUrl": "http://127.0.0.1:3000/flightjs/Flight/-/issues/39"
               }
             },
             {
               "name": "test2345",
               "type": "sum_type",
               "issue": {
                 "title": "Alias quia iure quibusdam vero nam dicta.",
                 "webUrl": "http://127.0.0.1:3000/flightjs/Flight/-/issues/18"
               }
             }
           ]
         }
       }
     }
   }