Graphing metrics and doing transformations/comparisons is (much) faster. Yes, metrics do have to be defined first but in my experience that's a non-issue since the things you want to monitor are usually immediately obvious during development (e.g. request-response times, errors returned, new customers acquired, pages loaded, etc.).
With that being said, it's not a mutually exclusive situation. You can have both. However, some logs used for plotting metrics have near-zero debugging value (e.g. a log line that just includes the timestamp and the message "event x occurred"). Those kinds of logs should be fully converted over to metrics.
Some other logs however are genuinely useful (e.g. an exception occurred, the error count should be incremented, and this is the stack trace).
With that being said, it's not a mutually exclusive situation. You can have both. However, some logs used for plotting metrics have near-zero debugging value (e.g. a log line that just includes the timestamp and the message "event x occurred"). Those kinds of logs should be fully converted over to metrics.
Some other logs however are genuinely useful (e.g. an exception occurred, the error count should be incremented, and this is the stack trace).