Base statistics represent the raw data collected. For example, counting the number of physical reads in the system since startup is a base statistic.

Metrics are secondary statistics derived from base statistics. Most metrics track the rates of change of activities in the Oracle database.
For example, the average physical reads in the system in the last 60 minutes is a metric. Metrics are used by internal client components for
system health monitoring, problem detection, and self-tuning.

For example, ADDM has a rule that requires the average physical reads in the system in the last 60 minutes. Another component may need a different metric based on the same base statistic:The Memory Advisor may need the physical read counts during peak hours. Oracle Database 10g supports metrics for system, session, file, and wait event statistics. Metrics are identified by a metric number. Each metric is also associated with a metric name. For more information about the metric-related views shown in the slide, see the Oracle Database Reference guide. The main benefit of keeping metrics is that when a component needs to compute the rate of change of some activities, the data is readily available. Previously, you had to capture statistics before and after having run your workload to compute the changed rate for a particular base statistic. With metrics, all you need to do is run your workload and select the corresponding metrics.

Note: Internally, MMON periodically updates metric data from the corresponding base statistics.Most of the metrics are refreshed every minute.

some dictionaries as follows:
V$SYSMETRIC,

V$SESSMETRIC,

V$SERVICEMETRIC,

V$METRICNAME,

V$FILEMETRIC,

V$EVENTMETRIC,
V$WAITCLASSMETRIC