Enable creation of custom performance metrics

[hub-bla]

Following PR contains:

• creation of register_performance_metric decorator to register custom metrics provided by user
• adjustment of performance_metrics to suite new way of function execution
• adjustment of test_performance_metrics to test custom metric
• documentation update for register_performance_metric usage

Closes Issue: #2234

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[tcuongd]

Nicely done, and with documentation too, thank you! Just one diff in the docs markdown that we should remove manually I think. Otherwise LGTM

[tcuongd]

Could you manually get rid of this diff pls, I assume it isn't supposed to change.