Traffic shows us how much demand is placed on the system, and is one of the The Four Golden Signals of Monitoring.
How you track traffic, largely depends on what type of service you are running.
For web services, traffic can be represented as a number of HTTP requests. It might be useful to break the traffic measurements down by request type – for example, static content requests and dynamic content requests.
For a real-time service, we could track network IO rate and number of concurrent sessions.
For worker services, we could track the number of jobs injected into the system.