Add some process metrics for GOOS=darwin

[makasim]

Adds support for process metrics on MacOS\Darwin. I added only those that do not require CGO.

• process_start_time_seconds
• process_cpu_seconds_total
• process_virtual_memory_max_bytes
• process_virtual_memory_bytes
• process_resident_memory_bytes
• process_open_fds
• process_max_fds

Fixes #75

Inspired by https://github.com/prometheus/client_golang/blob/main/prometheus/process_collector_darwin.go

[makasim]

@valyala asked for your review since the PR adds a lot of files to the vendor.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.68%. Comparing base (58ce999) to head (9a0a747).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #107   +/-   ##
=======================================
  Coverage   88.68%   88.68%           
=======================================
  Files          12       12           
  Lines        1388     1388           
=======================================
  Hits         1231     1231           
  Misses        106      106           
  Partials       51       51           

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[valyala]

@valyala asked for your review since the PR adds a lot of files to the vendor.

I don't want merging a pull request, which brings so big amounts of third-party code for the sake of exposing a few system metrics on MacOS. Please search for alternatives, which do not depend on hundreds of external code files. Is it possible to add the missing metrics by reading them from system files like it is done for these metrics on Linux at https://github.com/VictoriaMetrics/metrics/blob/master/process_metrics_linux.go ?