NewContext returns a logger that adds prefix before keyvals.

NewFilterLogger returns a Logger that accepts only log messages with "level" value <= level.

NewNopLogger returns a logger that doesn't do anything.

NewStdLogger returns logger based on standard "log" package.

Context is simplified version of go-kit log Context https://godoc.org/github.com/go-kit/kit/log#Context.

Logger is the fundamental interface for all log operations.