Add certification metric to OIT Monthly Metrics Report Job

[alisan16]

Connects #12748

Description

Add the final OIT monthly metric (certification usage) to the MonthlyMetricsReportJob so that the metric is reported to Slack. With this, someone on the Caseflow team can simply copy and paste the Slack message from #appeals-job-alerts to send to OIT, rather than needing to login to the production console to run the certification metric manually every month.

Also refactored spec/services/metrics/certification_usage_spec.rb so the same test cases can be reused

Acceptance Criteria

• Code compiles correctly
• Rspec tests pass

Testing Plan

Updated test spec/jobs/monthly_metrics_report_job_spec.rb
Run locally and printed out rather than sending to Slack:

[1] pry(main)> MonthlyMetricsReportJob.perform_now
Monthly report 2020-12-01 to 2020-12-31
Percentage of all cases certified with Caseflow: 100.0
Appeals established within 7 days: 0 (100%)
Supplemental Claims within 7 days: 0 (NaN%)
Higher Level Reviews within 7 days: 0 (NaN%)
type,total,in_progress,cancelled,processed,established_within_seven_days,established_within_seven_days_percent,median,avg,max,min
supplemental_claims,0,0,0,0,0,NaN,00:00:00,00:00:00,00:00:00,00:00:00
higher_level_reviews,0,0,0,0,0,NaN,00:00:00,00:00:00,00:00:00,00:00:00
request_issues_updates,0,0,0,0,0,NaN,00:00:00,00:00:00,00:00:00,00:00:00
Performed MonthlyMetricsReportJob (Job ID: fcfef14e-d829-463b-b2b8-ed43939eb4d9) from Shoryuken(caseflow_development_low_priority) in 536.47ms

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[yoomlam]

LGTM! Automation for the win!