[system/process] add support for mutlierr

[VihasMakwana]

What does this PR do?

• Previously, we weren't passing errors to the caller while monitoring set of processes.
  • With the recent introduction of the status reporter for metricsets, it is impossible to change the status to degraded if such errors are not passed to the caller.
• Fix this by passing errors to the caller. We also populate the process related information to our best-effort.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.md

Manual testing and general information

• See [metricbeat] - Allow metricsets to report their status via v2 protocol beats#40400 for testing it on metricbeat
  NOTE:
  • Only applicable if you're using system/process module
  • Non-fatal errors are only received when you have insufficient privileges.

Steps:

• While receiving any error, test for nature of error
• call errors.Is(err, NonFatalErr{})) on received error
  • If true, error is non-fatal and you can proceed further (metrics will be partially available, most probably insufficient privileges).
  • Else, log the error and stop execution (metrics will be empty)

Genreal info related to the changes in this PR:

• While getting process related information, you might also receive a non-nil error.

• Such errors come in two flavours:

  • Fatal errors:
    • This indicates that the error was fatal (for eg. no process found,)
    • Caller should stop further execution if they receive fatal errors
  • Non-fatal errors:
    • This indicates that the error was non-fatal (for eg. not enough privileges).
    • It means that metrics are partially filled.
    • Further execution can be continued if non-fatal errors are encountered

• Closes [metric/system/process] - return errors encountered while monitoring set of processes #164

[rdner]

Due to the nature of the change, we need to perform manual tests in order to ensure the change works. Please add instructions for manual testing to ensure updating to the new version of this library would not break Beats.

[VihasMakwana]

@rdner I've added the beats PR link in description