Extend DORA GraphQL API to support multiple metrics.

Summary

Current API supports only 1 metric per request. E.g.

{
  project(fullPath: "gitlab-org/gitlab") {
    dora {
      metrics(metric: LEAD_TIME_FOR_CHANGES) {
        date
        value
      }
    }
  }
}

while Value Streams Dashboard require multiple metrics to display on the same page. So we need a way to query multiple metrics in the same API request.

Proposal

1. Improve metrics resolver to accept multiple metrics via extended metric type.

{
  project(fullPath: "gitlab-org/gitlab") {
    dora {
      metrics {
        changeFailureRate
        date
        deploymentFrequency
        leadTimeForChanges
        timeToRestoreService
      }
    }
  }
}

2. Deprecate metric in favour of specific metric fields

/cc @blabuschagne