Add MarshalText and UnmarshalText for Duration

[metalmatze]

This is mostly needed for https://github.com/alecthomas/kong.
Furthermore it should be useful for others too, as it implements stdlib interfaces.

[roidelapluie]

model.Duration is an internal format, generally kong should use time.Duration. What would be the usecase?

[metalmatze]

time.Duration doesn't support anything above hours. Therefore it would need a custom wrapper to support 1w, 1y and more.
We want to add flags that support these time ranges too.
As there's no direct dependency on anything and this change implements two stdlib interfaces I don't see this hurting.

[roidelapluie]

I am not sure that I'd like to advise large projects like kong to rely on prometheus common given all the direct and indirect dependencies we have. We have complains like #255 already, and I do not want to amplify these. I agree that the code change is small, but it is also using exported interfaces only, such as this could as well be done downstream in kong with the same amount of code:

type promDuration model.Duration
* implement the marshallers

Would that be an option? (note that it would still have the issue pointed by #255)

[metalmatze]

There is no direct dependency with both projects anywhere. Kong won't depend on Prometheus common either. It depends on the encoding.TextUnmarshaler (https://golang.org/pkg/encoding/#TextUnmarshaler) which is an abstract interface in the stdlib. The only place where we would have both dependencies interact is in an application choosing to use kong and use model.Duration as flag data type.

package main

import (
	"fmt"

	"github.com/alecthomas/kong"
	"github.com/prometheus/common/model"
)

var CLI struct {
	Retention model.Duration `name:"retention" default:"15d" help:"Retention for our app."`
}

func main() {
	kong.Parse(&CLI)
	fmt.Printf("retention: %d\n", CLI.Retention)
}

Only in this downstream application both will be used. Before that there's no dependency with neither as kong depends on encoding.TextUnmarshaler.

[roidelapluie]

Okay, can you add a comment about which interface you are implementing?

[roidelapluie]

For the future it would have been great to check for error, even if we only return nil.

[roidelapluie]

Thanks!!