Add process metadataedit

The Add process metadata processor enriches events with information from running processes, identified by their process ID (PID).

- add_process_metadata:
    match_pids: [system.process.ppid]
    target: system.process.parent

The fields added to the event look as follows:

"process": {
  "name":  "systemd",
  "title": "/usr/lib/systemd/systemd --switched-root --system --deserialize 22",
  "exe":   "/usr/lib/systemd/systemd",
  "args":  ["/usr/lib/systemd/systemd", "--switched-root", "--system", "--deserialize", "22"],
  "pid":   1,
  "ppid":  0,
  "start_time": "2018-08-22T08:44:50.684Z",

Optionally, the process environment can be included, too:

  "env": {
    "HOME":       "/",
    "TERM":       "linux",
    "BOOT_IMAGE": "/boot/vmlinuz-4.11.8-300.fc26.x86_64",
    "LANG":       "en_US.UTF-8",

It has the following settings:

List of fields to lookup for a PID. The processor will search the list sequentially until the field is found in the current event, and the PID lookup will be applied to the value of this field.
(Optional) Destination prefix where the process object will be created. The default is the event’s root.
(Optional) List of fields to add. By default, the processor will add all the available fields except process.env.
(Optional) When set to false, events that don’t contain any of the fields in match_pids will be discarded and an error will be generated. By default, this condition is ignored.
(Optional) By default, if a target field already exists, it will not be overwritten and an error will be logged. If overwrite_keys is set to true, this condition will be ignored.
(Optional) By default, the process.env field is not output, to avoid leaking sensitive data. If restricted_fields is true, the field will be present in the output.