feat: Zenoss event handler

[alespour]

This PR adds Zenoss event handler as requested by https://github.com/influxdata/feature-requests/issues/72. Fixes #2483.

The handler provides options for Zenoss basic event fields:

handler option	event field	default value
summary	Summary	'{{ .Message }}'
device	Device	"" (empty string)
component	Component	"" (empty string)
severity	Severity	severity level (see Severity mapping)
eventClass	Event Class	"" (empty string)
eventClassKey	Event Class Key	"" (empty string)
collector	Collector (optional)	none
message	Message (optional)	none

In addition to the basic fields, custom fields can be set with customField option. The value of a custom field can be simple type or a string representation of JSON object. There are also handler options to override Zenoss event-related values set in the configuration: action, method, type, tid. All these options support templates with the following variables available: Name, TaskName, ID, Message, Details, Time, Duration, Level, Recoverable and Tags.

Severity mapping
Alert level is mapped to Zenoss event severity according to mapping specified in the configuration file. Severity can be specified either as a number or a string (see Event severity levels). The default configuration maps OK to Clear, Info to Info, Warning to Warning and Critical to Critical.

TICKscript examples:

stream
  |from()
    .measurement('cpu')
  |alert()
    .crit(lambda: "usage_user" > 90)
    .message('Hey, check your CPU')
    .zenoss()
        .device('{{ index .Tags "host" }}')
        .component('CPU')
        .eventClass('/App')

stream
  |from()
    .measurement('cpu')
  |alert()
    .crit(lambda: "usage_user" > 90)
    .message('Hey, check your CPU')
    .zenoss()
        .action('CustomRouter')
        .method('add_event')
        .device('{{ index .Tags "host" }}')
        .component('CPU')
        .eventClass('/App')
        .collector('localhost')
        .customField('ticks', 3300)

Default configuration:

[zenoss]
  # Configure Zenoss.
  enabled = false
  # The Zenoss URL.
  url = "https://tenant.zenoss.io/zport/dmd/evconsole_router"
  # Username for HTTP BASIC authentication
  # username = ""
  # Password for HTTP BASIC authentication
  # password = ""
  # Action (router name).
  action = "EventsRouter"
  # Router method.
  method = "add_event"
  # Event type.
  type = "rpc"
  # Temporary request transaction ID.
  tid = 1
  # Alert level to event severity mapping.
  severity-map = { OK = "Clear", Info = "Info", Warning = "Warning", Critical = "Critical" }

Required for all non-trivial PRs

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated

[docmerlin]

@alespour :Can you move this to try to merge with v1.6-master instead of pointing at current master?

[alespour]

@docmerlin Done.