Add security event metrics to custom documentation

[AsuNa-jp]

Change Summary

• https://github.com/elastic/endpoint-dev/pull/16755

This PR adds new metrics fields for the security events.

Endpoint.metrics.documents_volume.security_events.sources.sent_bytes
Endpoint.metrics.documents_volume.security_events.sources.sent_count
Endpoint.metrics.documents_volume.security_events.sources.source
Endpoint.metrics.documents_volume.security_events.sources.suppressed_bytes
Endpoint.metrics.documents_volume.security_events.sources.suppressed_count

Release Target

9.2

For mapping changes:

• I ran make after making the schema changes, and committed all changes