Rename metrics

[loelkes]

I would like to rename and restructure the metrics so that the naming corresponds more to the OctoPrint namespace and follows some Prometheus best practices

1. Add units where they are fixed.
2. Follow Octoprint namespace. For example use keyword job instead of print for print jobs.
3. Organize in categories printer, job, jobs and server

Examples:

• octoprint_z_travel_print becomes octoprint_job_travel_z
• octoprint_z_travel_total becomes octoprint_printer_travel_z
• octoprint_done_prints becomes octoprint_jobs_done
• octoprint_extrusion_print becomes octoprint_job_extrusion
• octoprint_extrusion_total becomes octoprint_printer_extrusion
• octoprint_infos becomes octoprint_server_info

No units to be set for distances and temperatures since Marlin (and others?) support Fahrenheit or Inches as well IIRC.

This would lead to version 3, since it is a breaking change. Please let me know if you @tg44 agree with the global direction and I'll prepare a PR.

[tg44]

I think its a good direction.