Logging: avoid expensive formatting when level is disabled.

[bboreham]

This function:

common/logging/gokit.go

Lines 39 to 41 in 53b7240

	func (g gokit) Debugf(format string, args ...interface{}) {
	level.Debug(g.Logger).Log("msg", fmt.Sprintf(format, args...))
	}

will do the printf with all string formatting, memory allocation, etc., regardless of whether debug logging is enabled.

This appears to be the intention of go-kit/log/level; it does not expose any way to ask about levels outside of Log().
We could keep a note of which level is allowed, thus shortcut the Sprintf call above.

[bboreham]

I think this was fixed by #227.