While a lot of our logging needs seem like they would be fulfilled by this system — because we attach trace IDs to log messages, and because (at least in Payments) you can usually find the appropriate trace ID by searching for a Payment ID, which could be annotated too — there are definitely many times I've copy/pasted the text in quotes from a log-generating line of code in a Java or Go file, to find out if it's being executed, or as a handle into a subsection of code/logging.
In the linked design doc, you include a motivating tweet near the top, saying, “just give me log files and grep, I am dying”. But unless I'm misreading things, there's no `grep` here. Right?
I'm guessing you could narrow down (using metadata) and then grep, but if the narrowest metadata you have is app name and time range, you're still going to be grepping over a lot of data…