feat: ServiceNow event handler

[alespour]

This PR adds ServiceNow event handler as requested by https://github.com/influxdata/feature-requests/issues/176.

The handler supports several string type options that can be set when using .serviceNow() in a TICKscript which are used to set corresponding fields in ServiceNow alert or event (see https://docs.servicenow.com/bundle/paris-it-operations-management/page/product/event-management/task/t_EMManuallyCreateAlert.html)

handler option	alert/event field
node	Node
type	Type
resource	Resource
metricName	Metric name
messageKey	Message key

All these options support templates with the following variables available: ID, Name, TaskName, Fields, Tags (with the same meaning as in AlertNode.message)

By default, the handler maps config and/or event values to ServiceNow Alert or Event fields as follows:

value	field
source	Source
message	Description
alert ID	Message key

TICKscript examples:

stream
  |from()
    .measurement('cpu')
  |alert()
    .crit(lambda: "usage_user" > 90)
    .stateChangesOnly()
    .message('Hey, check your CPU')
    .serviceNow()

stream
  |from()
    .measurement('cpu')
  |alert()
    .crit(lambda: "usage_user" > 90)
    .message('Hey, check your CPU')
    .serviceNow()
        .node('{{ index .Tags "host" }}')
        .type('CPU')
        .resource('CPU-Total')
        .metricName('usage_user')
        .messageKey('Alert: {{ .ID }}')

Default configuration:

[servicenow]
  # Configure ServiceNow.
  enabled = false
  # The ServiceNow URL for target table (Alert or Event). Replace instance with actual hostname.
  url = "https://instance.service-now.com/api/now/v1/table/em_alert"
  # Default source identification.
  source = "Kapacitor"
  # Username for HTTP BASIC authentication
  # username = ""
  # Password for HTTP BASIC authentication
  # password = ""

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• Rebased/mergable
• Tests pass
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N/A

ServiceNow alert with default options:

ServiceNow alert with custom options:

[docmerlin]

@alespour has this been tested against ServiceNow? I don't have an account to test this. Otherwise it looks fine.

[docmerlin]

This looks fine so long as it has been manually tested with ServiceNow.

[alespour]

@docmerlin Yes it was tested manually, the screenshots show alerts sent from Kapacitor.