Add Bitbucket Server Importer dashboard

This MR adds a new Bitbucket Server Importer dashboard under sidekiq folder which tracks the following:

• Number of importer projects per hour
• Number of importer merge (pull) requests per hour
• Number of importer issues per hour
• Total number of imported projects
• Total number of imported merge (pull) requests
• Total number of importer issues
• Project import duration

It also:

• Updates Bitbucket Importer dashboard, as counter metric names were updated to match standard naming convention
• Update metric names that mention pull_requests to merge_requests, so that we can use shared metrics module in rails for other importers

Related gitlab MR: gitlab-org/gitlab!33700 (merged) Previous Bitbucket Importer dashboard MR: !2125 (merged)