As of the Spring '19 release you are now able to monitor Event Delivery through the limits resource via the REST API (stored in the DailyStandardVolumePlatformEvents response) - https://releasenotes.docs.salesforce.com/en-us/spring19/release-notes/rn_messaging_standard_volume_limit.htm
To monitor your monthly event delivery usage for High Volume Platform Events if you have purchased the add-on, use the limits REST API resource, and inspect the MonthlyPlatformEvents value. With the add-on purchase, they measure entitlement usage over the month rather than the strictly enforcing the daily limit. The limits resource returns the monthly maximum allocation - https://developer.salesforce.com/docs/atlas.en-us.platform_events.meta/platform_events/platform_event_limits.htm