[cmd/mdatagen] Add definition of events to metadata-schema.yaml (#13058)

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#### Description
Add definition of `events` to `metadata-schema.yaml`

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#### Link to tracking issue
Part of #12571 

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#### Testing
n/a

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#### Documentation
Added

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---------

Co-authored-by: Dmitry Anoshin <[email protected]>

Author: sincejune

cmd/mdatagen/metadata-schema.yaml

@@ -117,6 +117,28 @@ metrics:
     # Optional: array of attributes that were defined in the attributes section that are emitted by this metric.
     attributes: [string]
 
+# Optional: map of event names with the key being the event name and value
+# being described below.
+events:
+  <event.name>:
+    # Required: whether the event is collected by default.
+    enabled: bool
+    # Required: event description.
+    description:
+    # Optional: extended documentation of the event.
+    extended_documentation:
+    # Optional: warnings that will be shown to user under specified conditions.
+    warnings:
+      # A warning that will be displayed if the event is enabled in user config.
+      # Should be used for deprecated default events that will be removed soon.
+      if_enabled:
+      # A warning that will be displayed if `enabled` field is not set explicitly in user config.
+      if_enabled_not_set:
+      # A warning that will be displayed if the event is configured by user in any way.
+      if_configured:
+    # Optional: array of attributes that were defined in the attributes section that are emitted by this event.
+    attributes: [string]
+
 # Lifecycle tests generated for this component.
 tests:
   config: # {} by default, specific testing configuration for lifecycle tests.