document what each metric means

[yob]

• pool_capacity the number of idle and unused worker threads. When this is low/zero, puma is running at full capacity and might need scaling up puma/puma#1579
• backlog the number of requests that have made it to a worker but are yet to be processed. This will normally be zero, as requests queue on the tcp/unix socket in front of the master puma process, not in the worker thread pool puma/puma#1577
• max_threads - the maximum number of worker threads that might run at one time.
• running - the number of worker threads currently running. In quiet times, idle threads may be shutdown, but they'll start back up again when traffic arrives
• requests_count - incrementing count of handled requests puma/puma#2106
• workers - the number of worker processes
• booted_workers
• old_workers - puma/puma#860