Ship additional GitLab Prometheus alerts

We will be shipping AlertManager via GitLab 10.8 (omnibus-gitlab#2999 (closed)). I think we should begin shipping default alerts for GitLab administrators. What metrics are most useful to add ASAP as alerts for most GitLab users/customers?

Some ideas:

Component	Exporter Endpoint	Prometheus metric
Unicorn	http://localhost:8080/-/metrics	unicorn_active_connections
Unicorn	http://localhost:8080/-/metrics	unicorn_queued_connections
Unicorn	http://localhost:8080/-/metrics	job_register_attempts_failed_total
Sidekiq	http://localhost:9168/sidekiq	sidekiq_queue_size
Gitaly	http://localhost:9236/metrics	grpc_server_handled_total{grpc_code="ResourceExhausted}
PostgreSQL	http://localhost:9187/metrics	pg_stat_database_deadlocks
PostgreSQL	http://localhost:9187/metrics	pg_stat_database_conflicts_confl_deadlock
Redis	http://localhost:9121/metrics	redis_up
Workhorse	http://localhost:9229/metrics	?
Pages	http://localhost:9101/metrics	gitlab_pages_domains_updated_total

/cc: @ayufan, @bjk-gitlab, @nick.thomas, @_stark, @dblessing, @jacobvosmaer-gitlab, @zj