This change allows Registries to be used as Collectors.

This enables new instances of Registry to be passed to ephemeral
subroutines for collecting metrics from subroutines which are still
running:

```go
package main

import (
  "fmt"

  "github.com/prometheus/client_golang/prometheus"
)

func main() {
  globalReg := prometheus.NewRegistry()

  for i := 0; i < 100; i++ {
    workerReg := prometheus.WrapRegistererWith(prometheus.Labels{
      // Add an ID label so registered metrics from workers don't
      // collide.
      "worker_id": fmt.Sprintf("%d", i),
    }, prometheus.NewRegistry()

    globalReg.MustRegister(workerReg)

    go func(i int) {
      runWorker(workerReg)

      // Unregister any metrics the worker may have created.
      globalReg.Unregister(workerReg)
    }(i)
  }
}

// runWorker runs a worker, registering worker-specific metrics.
func runWorker(reg *prometheus.Registry) {
  // ... register metrics ...
  // ... do work ...
}
```

This change makes it easier to avoid leaking metrics from subroutines
which do not consistently properly unregister metrics.

Signed-off-by: Robert Fratto <robertfratto@gmail.com>

Signed-off-by: Robert Fratto <robertfratto@gmail.com>

Signed-off-by: Robert Fratto <robertfratto@gmail.com>