What is a metric?

A metric is any type of numerical variable used to gauge some quantifiable measure of performance of a type of object (server, application, business, sensor, etc.) that can be monitored. Examples of metrics: requests/sec. serviced by an application, CPU load, stock price, temperature, number of website visits.

A metric can have more than one data stream aka metric stream associated with it. For example if you are tracking CPU from 100 servers, the CPU metric would have 100 data streams associated with it, one for each server.