[Monitor] Add Public Preview APIs of Metric Baseline

[AutorestCI]

Generated from RestAPI PR: Azure/azure-rest-api-specs#2049

[lmazuel]

LGTM to me, that's new operations.

[codecov-io]

Codecov Report

Merging #1710 into master will decrease coverage by 0.03%.
The diff coverage is 41.39%.

@@            Coverage Diff             @@
##           master    #1710      +/-   ##
==========================================
- Coverage   55.33%   55.29%   -0.04%     
==========================================
  Files        4202     4215      +13     
  Lines      100158   100436     +278     
==========================================
+ Hits        55422    55540     +118     
- Misses      44736    44896     +160

Impacted Files	Coverage Δ
...or/azure/mgmt/monitor/models/recurrent_schedule.py	50% <0%> (-5.56%)	⬇️
...-monitor/azure/mgmt/monitor/models/scale_action.py	50% <0%> (-5.56%)	⬇️
...-monitor/azure/mgmt/monitor/models/sms_receiver.py	50% <0%> (-5.56%)	⬇️
...t/monitor/models/activity_log_alert_action_list.py	66.66% <0%> (-13.34%)	⬇️
...mgmt-monitor/azure/mgmt/monitor/models/incident.py	45.45% <0%> (-4.55%)	⬇️
...iagnostic_settings_category_resource_collection.py	66.66% <0%> (-13.34%)	⬇️
...itor/azure/mgmt/monitor/models/retention_policy.py	62.5% <0%> (-8.93%)	⬇️
...onitor/azure/mgmt/monitor/models/email_receiver.py	55.55% <0%> (-6.95%)	⬇️
...or/azure/mgmt/monitor/models/email_notification.py	50% <0%> (-7.15%)	⬇️
.../monitor/models/activity_log_alert_action_group.py	62.5% <0%> (-8.93%)	⬇️
... and 81 more

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