Remove Rails background transaction metrics

Sean McGivernrequested to merge
remove-rails-background-transaction-metrics into master

What does this MR do?

Remove Rails background transaction metrics. These are covered by Sidekiq-specific metrics in https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/sidekiq_middleware/server_metrics.rb, and increase our metric cardinality.

For gitlab-com/gl-infra/scalability#517.

Edited by Sean McGivern

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