Custom exposition of the Metrics

[oliverpool]

The prometheuscollector sub-package gives a short example to make a custom exposition of the metrics:

handler, err := tusd.NewHandler(…)
collector := prometheuscollector.New(handler.Metrics)
prometheus.MustRegister(collector)

It does however not work when used in an other package (main in my case), because github.com/prometheus/client_golang/prometheus is vendored inside tusd:

cannot use collector (type prometheuscollector.Collector) as type "github.com/prometheus/client_golang/prometheus".Collector in argument to "github.com/prometheus/client_golang/prometheus".MustRegister:
	prometheuscollector.Collector does not implement "github.com/prometheus/client_golang/prometheus".Collector (wrong type for Collect method)
		have Collect(chan<- "github.com/tus/tusd/vendor/github.com/prometheus/client_golang/prometheus".Metric)
		want Collect(chan<- "github.com/prometheus/client_golang/prometheus".Metric)

If I try to import "github.com/tus/tusd/vendor/github.com/prometheus/client_golang/prometheus" instead, it fails with use of vendored package not allowed.

[oliverpool]

A possible solution is to add the following function to prometheuscollector.go:

// Handler creates a prometheus handler to serve the metrics
func Handler(metrics *tusd.Metrics) http.Handler {
	prometheus.MustRegister(New(metrics))
	return prometheus.Handler()
}

But it does not allow to add custom metrics.

[Acconut]

We encountered the exact same problem with the S3Store and ConsulLocker structs. The only proper way to address this is to actually remove the packages from the vendor directory. For example, you will not find github.com/aws/aws-go-sdk or github.com/hashicorp/consul there. I will probably go with a same route to tackle this issue.

[oliverpool]

I will probably go with a same route to tackle this issue.

Do you want a PR for this?

[Acconut]

Thank you for offering your help here! I took care of it now because it also requires a few adjustments using govendor (our dependency manager) and the test scripts for Travis/AppVeyor. Please try the above commit :)