Add support for CustomSchedule option in DatadogMonitor

[rafanaskin]

What does this PR do?

Support the customSchedule option (rrule,  timezone and start) in DatadogMonitor CRD.

Motivation

To implement the feature request

Additional Notes

N/A

Minimum Agent Versions

N/A

Describe your test plan

I've tested it locally with this manifests.

apiVersion: datadoghq.com/v1alpha1
kind: DatadogMonitor
metadata:
  name: datadog-monitor-test
  namespace: system
spec:
  query: "avg(last_5m):avg:system.load.1{*} > 4"
  type: "query alert"
  name: "Test monitor made from DatadogMonitor"
  message: "1-2-3 testing"
  priority: 2
  options:
    includeTags: false
    thresholds:
      critical: "4"
    schedulingOptions:
      customeSchedule:
        recurrence:
          rrule: "FREQ=MONTHLY;BYMONTHDAY=2"
          timezone: "America/New_York"

apiVersion: datadoghq.com/v1alpha1
kind: DatadogMonitor
metadata:
  name: datadog-monitor-test
  namespace: system
spec:
  query: "avg(last_5m):avg:system.load.1{*} > 4"
  type: "query alert"
  name: "Test monitor made from DatadogMonitor"
  message: "1-2-3 testing"
  priority: 2
  options:
    includeTags: false
    thresholds:
      critical: "4"
    schedulingOptions:
      customeSchedule:
        recurrence:
          rrule: "FREQ=MONTHLY;BYMONTHDAY=2"
          timezone: "America/New_York"
          start: "2025-06-02T00:00:00"

apiVersion: datadoghq.com/v1alpha1
kind: DatadogMonitor
metadata:
  name: datadog-monitor-test
  namespace: system
spec:
  query: "avg(last_5m):avg:system.load.1{*} > 4"
  type: "query alert"
  name: "Test monitor made from DatadogMonitor"
  message: "1-2-3 testing"
  priority: 2
  options:
    includeTags: false
    thresholds:
      critical: "4"
    schedulingOptions:
      customeSchedule:
        recurrence:
          rrule: "INVALID VALUE"
          timezone: "America/New_York"

Checklist

• [ ] PR has at least one valid label: bug, enhancement, refactoring, documentation, tooling, and/or dependencies
• [ ] PR has a milestone or the qa/skip-qa label

[levan-m]

@rafanaskin thanks for your contribution! I set 1.17 milestone, we will review and provide feedback in next few weeks.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 46.51%. Comparing base (b1a1884) to head (ba6e6a9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1949      +/-   ##
==========================================
+ Coverage   46.47%   46.51%   +0.03%     
==========================================
  Files         248      248              
  Lines       24687    24704      +17     
==========================================
+ Hits        11473    11490      +17     
  Misses      12629    12629              
  Partials      585      585              

Flag	Coverage Δ
unittests	46.51% <100.00%> (+0.03%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
internal/controller/datadogmonitor/monitor.go	73.24% <100.00%> (+2.15%)	:arrow_up:

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[rafanaskin]

Thank you @tbavelier, and I apologise for the typos.