Add support for defining internal metrics

Related to https://issues.apache.org/jira/browse/LOG4J2-2937 (provide counters for logger rates) and #1927, we need some sort of basic metrics API we can use internally for exporting said metrics into a metrics library. This should not be created as brand new metrics API (if we want to do that, then we'd probably start somewhere like having additional modules which implement various OpenTelemetry APIs for better reusability), but it does need to define enough of an API so that various metrics collectors can use this. I've seen some similar ideas before in various Spring libraries before Spring Framework 6 integrated Micrometer everywhere as the general metrics API, so there may be some inspiration to take.