Upgrade gitlab-monitor to 3.1.0

A number of improvements came from upgrading 2.19.1 to 3.1.0:

• Fix Sidekiq metric names
  1. sidekiq_queue_latency -> sidekiq_queue_latency_seconds
• Fix PostgreSQL metric names
  1. pg_stat_table_* -> gitlab_database_stat_table_*
  2. db_rows_count -> gitlab_database_rows
• Simplify use by reading all stats from /proc/<pid>/stat.
• Simplify metrics for total resident and virtual bytes.
• Use process start time to fit with Prometheus conventions.
• Add process CPU seconds metric.
• Handle Process.clock_getres failures.
• Fix git-upload-pack processes always showing 1 process
• Add support for parsing process smaps
• Gem dependency updates
  1. pg 0.18.4 -> 1.1
  2. sinatra 1.4.7 -> 2.0.4
  3. rack 1.6.1 -> 2.0.6
  4. redis-namespace 1.5.2 -> 1.6.0
• Provide Proportional Size Set (PSS) for Unicorn and Sidekiq