metrics.log is measuring the thruput of data being actually being indexed by Splunk, as a measure of how well your input and indexing pipelines are performing. The metrics.log file itself is indeed indexed to the _internal index because you can run a splunk search and have it show up.
However, this data and the other data indexed by Splunk about Splunk in _internal and _introspection and a few other indexes, does not actually count toward your license. Additionally data that is indexed by Splunk out of summarization queries run against other Splunk data and written into Summary Indexes is additionally not counted toward your license, however it is possible to configure your Splunk Server(s) to have inputs of their own and pick up data that isn't about Splunk itself, thus would actually count toward your license.
To figure out actual license impact (instead of performance metrics) you'll want to look on your license master, there should be a search called the "License Usage Data Cube" which helps build breakdowns and the License Usage Report View which will let you see the actual license impact against various indexes and hosts. (You should read the documentation page because there is squashing behavior that could take place in the data sent to the license master from each indexer.
