Add load balancing Prometheus metrics in server_metrics middleware

doc/administration/monitoring/prometheus/gitlab_metrics.md

...elogs/unreleased/326225-add-sidekiq-lb-server-metrics.yml

ee/lib/ee/gitlab/sidekiq_middleware/server_metrics.rb

...tlab/database/load_balancing/sidekiq_client_middleware.rb

...tlab/database/load_balancing/sidekiq_server_middleware.rb

...c/lib/ee/gitlab/sidekiq_middleware/server_metrics_spec.rb

lib/gitlab/sidekiq_middleware/instrumentation_logger.rb

lib/gitlab/sidekiq_middleware/server_metrics.rb

spec/lib/gitlab/sidekiq_middleware/client_metrics_spec.rb

.../gitlab/sidekiq_middleware/instrumentation_logger_spec.rb

spec/lib/gitlab/sidekiq_middleware/server_metrics_spec.rb

spec/lib/gitlab/sidekiq_middleware_spec.rb

...itlab/sidekiq_middleware/server_metrics_shared_context.rb

...ics_middleware_with_worker_attribution_shared_examples.rb