feat(parsers.xpath): Allow to specify byte-array fields to encode in HEX

[srebhan]

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

resolves #11952

This PR adds the last missing part for supporting PowerDNS protocol-buffer messages. To do this, the PR adds an ability to specify byte-array fields (via fields_bytes_as_hex) that should be represented as HEX in the xpath parser. This is necessary to be able to handle IP addresses and IDs correctly which would otherwise encoded as UTF-8 strings which are hard to reverse. The following config

[[inputs.socket_listener]]
  service_address = "tcp://127.0.0.1:8000"
  splitting_strategy = "variable length"
  splitting_length_field = {offset = 0, bytes = 2, endianness = "BE", header_length=2}

  data_format = "xpath_protobuf"

  xpath_native_types = true
  xpath_protobuf_file = "powerdns_message.proto"
  xpath_protobuf_type = "PBDNSMessage"
  xpath_protobuf_import_paths = [".","test_configs"]

  [[inputs.socket_listener.xpath]]
    fields_bytes_as_hex = ["from", "to", "messageId", "originalRequestorSubnet"]
    field_selection = "descendant::*"

[[processors.starlark]]
  source = '''
load("time.star", "time")

def ip(s):
  if len(s) == 8:
    # IPv4 addresses
    return '.'.join([str(int(s[i:i+2], base=16)) for i in range(0, len(s), 2)])

  # IPv6 addresses
  return ':'.join([s[i:i+4] for i in range(0, len(s), 4)])

def apply(metric):
  # Convert IP addresses
  ip_fields = ["from", "to", "originalRequestorSubnet"]
  for k in ip_fields:
    if k in metric.fields:
      metric.fields[k] = ip(metric.fields[k])

  # Extract the time
  t = metric.fields.pop("timeSec")*1000000000 + metric.fields.pop("timeUsec")*1000
  metric.time = time.from_timestamp(0, t).unix_nano

  return metric
'''

allows to send a message to the socket_listener input and retrieve a metric in the form

powerdns from="7f000001",fromPort=45729u,to="7f000001",toPort=53u,inBytes=48u,serverIdentity="xxxxxxxxxxxxxxxxxxx.com",messageId="943f90bea57a4eecbc5b0bea820a8aae",qName="ilse.nl.",qClass=1u,qType=1u,id=64100u,timeSec=1665050957u,timeUsec=500976u

[powersj]

per tests: sounds like you need a make docs

[telegraf-tiger]

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