StdLibAdapter can not extract log line with set prefix correctly

[soven]

When using a prefix in stdlib's logger StdLibAdapter extracts the fields from log line incorrectly.

For example, the program

package main

import (
    "log"
    "os"

    gokitlog "github.com/go-kit/kit/log"
)

func main() {
    logg := log.New(gokitlog.NewStdlibAdapter(gokitlog.NewLogfmtLogger(os.Stdout)),
        "some prefix ", log.LstdFlags)
    logg.Println("log line")
}

will give us the following to the stdout:

caller="some prefix 2020/11/16 22:33" msg=":04 log line"

I have a fix to support the prefix mode by adding a method Prefix(prefix string, joinPrefixToMsg bool) StdlibAdapterOption

[ChrisHines]

We would be happy to review a PR to fix this issue.

[soven]

done

[sagikazarmark]

Fixed in #1036