promlog: check the log level first

[royeo]

The logger returned by level.NewFilter will check the log level first, this avoids meaningless caller and ts calls.

[roidelapluie]

Can you please explain why we would need this change? Thanks!

[royeo]

The logger returned by log.With will gets the value of the bound field such as ts and caller, runtime.Caller is expensive. So if it's the previous logic, logger will obtains the binding value before checking the log level, this has some impact on performance.

[roidelapluie]

Thank you, this is correct.

[bboreham]

Looks like this breaks printing the caller. DefaultCaller is hard-coded to select the 3rd stack frame up, and the filter adds another one.

https://play.golang.org/p/FPPgJPuk2L3