Test metrics are quantitative measures that are used to monitor, control, and improve the test process and its outcomes. Test metrics can be collected at different levels of testing (test case, test suite, test project, etc.) and can be used for different purposes (planning, estimation, execution, evaluation, etc.). Some examples of common test metrics are:

Percentage of work done in test environment creation: This metric indicates how much effort has been spent on setting up and maintaining the test environment, which includes hardware, software, network, data, tools, etc., that are required for conducting the test activities.

Number of test cases run: This metric indicates how many test cases have been executed during a given period or phase of testing.

Deviation from test milestone dates: This metric indicates how much delay or ahead of schedule the test activities are compared to the planned dates.

Defect density: This metric indicates how many defects have been found per unit of size or functionality of the system under test.

Average number of expected defects per requirement is not a common test metric because it is not easy to estimate or measure how many defects are likely to be found for each requirement. Moreover, this metric does not provide useful information for improving the test process or evaluating the test results. You can find more information about test metrics in A Study Guide to the ISTQB® Foundation Level 2018 Syllabus, Chapter 5, Section 5.41.