Add metrics report type to JobArtifact constants

Add sample metrics fixture

Add factories for artifacts with metrics reports

Adds EE spec factories for CI models that can have metrics reports.

Add scopes for CI models with metrics reports

Adds scopes with metrics reports for Pipeline, Builds and JobArtifacts.

Add metrics_reports Premium feature

Implement has_metrics_reports in Pipelines and MRs

Implements EE::Ci::Pipeline#has_metrics_reports? and
EE::MergeRequest#has_metrics_reports?.

Implement metrics CI Report and Parser

Implements Parser and Report types for metrics reports.

Expose metrics reports to the Merge Request

Implements EE::Ci::Build#collect_metrics_reports! and
EE::Ci::Pipeline#metrics_report and exposes metrics report for
consumption by the Merge Request.

Implement metrics reports comparer

Implements a class that compares two metrics reports and selects new,
existing and removed metrics.

Implement Metric report serializer

Implements a serializer for metrics reports to be used in the metrics
reports comparison service.

Implement metrics reports comparison service

Implements Ci::CompareMetricsReportsService that compares two
metrics reports and serializes the result for frontend consumption.

Expose metrics reports comparison in Merge Request

Add Merge Requests controller endpoint for metrics

Implements a Merge Request controller endpoint for querying metrics
reports.