Minor: Surface "unlabeled" in a way that can be referenced

[henricook]

I'm using a Doobie 1.x loghandler to expose execution times for my queries via a prometheus metrics endpoint from my application.

I only log queries that have a label (otherwise I wouldn't know what to call them), currently to check for a label in the logHandler I have to do:

if (label =!= "unlabeled") {

I'd be nice if I could get the "unlabeled" value from Doobie directly to defend against future changes and save the 'magic string'

It'd be better if label was an Option, but I appreciate that would probably be a significant breaking change!

[henricook]

Thanks @jatcwang @felix-hedenstrom !