Taking into account that being a developer requires a really broad range of competencies (from "hard" knowledge about languages, frameworks, databases, systems

Taking into account that being a developer requires a really broad range of competencies (from "hard" knowledge about languages, frameworks, databases, systems and so long, through a variety of skills like analyzing, solving problems etc. to "soft" aspects communicating and cooperating with others, not to mention the business domain requirements which often are crucial) I think you should first focus on finding one narrow area you would like to improve and then seek for a right metric for it. How to choose it? It depends on the reason why you would like to improve: to be more attractive on the job market? To spend less time coping with specific problems? To earn more money? To gain recognition for your work? To have more personal satisfaction from what you do and how you do it? etc. Knowing the "ultimate" goal can help decide what exactly you want to focus on, establish a baseline and measure the progress.