feat(metrics): add process collector

[flaneur2020]

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Summary

fixes #15538

this PR adds the following process metrics, they're considered as the standard process metrics:

# HELP process_cpu_seconds_total Total user and system CPU time spent in seconds.
# TYPE process_cpu_seconds_total counter
process_cpu_seconds_total 138100.24
# HELP process_max_fds Maximum number of open file descriptors.
# TYPE process_max_fds gauge
process_max_fds 1.048576e+06
# HELP process_open_fds Number of open file descriptors.
# TYPE process_open_fds gauge
process_open_fds 26
# HELP process_resident_memory_bytes Resident memory size in bytes.
# TYPE process_resident_memory_bytes gauge
process_resident_memory_bytes 3.7982208e+07
# HELP process_start_time_seconds Start time of the process since unix epoch in seconds.
# TYPE process_start_time_seconds gauge
process_start_time_seconds 1.70893894953e+09
# HELP process_virtual_memory_bytes Virtual memory size in bytes.
# TYPE process_virtual_memory_bytes gauge
process_virtual_memory_bytes 1.346695168e+09
# HELP process_virtual_memory_max_bytes Maximum amount of virtual memory available in bytes.
# TYPE process_virtual_memory_max_bytes gauge
process_virtual_memory_max_bytes 1.8446744073709552e+19

as prometheus/rust-client still has not builtin support for process collector, this PR adds a vanilla implementation which only works in linux. we can switch to the builtin implementation after prometheus/client_rust#29 got resolved.

Tests

• Unit Test

Type of change

• Other (please describe): metrics improvement

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This change is 

[zhang2014]

Perhaps it can be add process level metrics when reading system.metrics.

[flaneur2020]

Perhaps it can be add process level metrics when reading system.metrics.

we need find a way to let system.metrics able to expose the metrics from Collector, maybe we can add it in another PR to make a refactor about it.