Enable and use Activity Logging in Dynamics 365 https://docs.microsoft.com/en-us/dynamics365/customer-engagement/admin/enable-use-comprehensive-auditing
Use Common Data Service for Apps analytics to view metrics about your online instance https://docs.microsoft.com/it-it/power-platform/admin/analytics-common-data-service
Manage storage for Microsoft Dynamics 365 (online) https://technet.microsoft.com/en-us/library/jj191611.aspx
Manage Dynamics 365 (online) Sandbox instances https://technet.microsoft.com/en-us/library/dn659833.aspx