Looking for procedure/link/document for Logging of transactions/ status codes from Apigee X environment - API services in GCP and publish to observability tools such as Splunk, Coralogix etc. using Pub/Sub model for data ingestion.
You should first take a look at using the Message Logging policy. You can add this in your api proxy, and have it send data to a syslog endpoint directly and you can customize what is logged to suit your needs.
please elaborate more about syslog endpoint (as not looking for REST API method of implementation)and initiate Pub/Sub model from GCP perspective (https://cloud.google.com/pubsub?hl=en)??)