Idea: 'Helper' method for Caller

Currently, it is necessary to figure out how each log method will be called in order to specify a count of stack frames to `Caller`.
https://github.com/go-kit/log/blob/5739c2646c7293c773ed50d2086e843e3330781c/value.go#L82-L84

This makes functions that wrap or forward to `go-kit/log` hard to use.

Suggestion: have a [Helper](https://pkg.go.dev/testing#T.Helper) method like in `testing`, which tells `Caller` to skip that function from the count.

This would be relatively expensive – a stack walk, lock and map lookup each time the helper is entered, and more map lookups when actually logging – but much more pleasant to use.