[service/proctelemetry] Offer to override the HOST_PROC environment variable with a programmatic value

[atoulme]

Reprising #7434

Description:

While debugging the below error in k8s env

Error: failed to register process metrics: process does not exist
2023/03/23 03:44:47 main.go:115: application run finished with error: failed to register process metrics: process does not exist

I have noticed that the metric server is calling GOPSUTIL while the HOST_PROC variable is set , this causes gopsutil PidExistsWithContext to retrieve the process from the host instead from the container

func PidExistsWithContext(ctx context.Context, pid int32) (bool, error) {
	if pid <= 0 {
		return false, fmt.Errorf("invalid pid %v", pid)
	}
	proc, err := os.FindProcess(int(pid))
	if err != nil {
		return false, err
	}

	if isMount(common.HostProc()) { // if /<HOST_PROC>/proc exists and is mounted, check if /<HOST_PROC>/proc/<PID> folder exists
		_, err := os.Stat(common.HostProc(strconv.Itoa(int(pid))))
		if os.IsNotExist(err) {
			return false, nil
		}
		return err == nil, err
	}

This PR unsets and resets the host_proc variable and introduces an option to allow the use of host_proc if for whatever reason they need to

Link to tracking Issue:
Fixes #7435

Testing:
unit tests

[dloucasfx]

LGTM !!! the tests are failing but not related to the change

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.34% ⚠️

Comparison is base (9375d95) 90.82% compared to head (2e3f175) 90.49%.
Report is 38 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7998      +/-   ##
==========================================
- Coverage   90.82%   90.49%   -0.34%     
==========================================
  Files         300      301       +1     
  Lines       15108    15727     +619     
==========================================
+ Hits        13722    14232     +510     
- Misses       1112     1207      +95     
- Partials      274      288      +14     

Files Changed	Coverage Δ
...ervice/internal/proctelemetry/process_telemetry.go	85.07% <100.00%> (+1.20%)	⬆️

... and 20 files with indirect coverage changes

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