Add factory for creating DogStatsd sinks from a prexisting datadog client

[bobbytables]

As of right now, the datadog metric sink does not use buffered flushes, which can cause a noticeable slowdown in emitting metrics. The DogStatsd package offers a buffered version of their client, but because the client is not exported from this sink, it's not possible to set it. This change allows you to pass in a client that you initialize.

For example, it would be nice to be able to use the buffered client like so:

package foo

import (
  "github.com/DataDog/datadog-go/statsd"
  "github.com/armon/go-metrics/datadog"
)

client, err := statsd.NewBuffered("localhost:8125", 1024)
if err != nil {
  panic(err)
}

sink := datadog.NewDogStatsdSinkFromClient(client, "hostname")

[bobbytables]

Any thoughts on this?

[aantono]

Would it be possible, as part of this PR, to also add a getter to the DogStatsD sync to expose the Client? Since the DogStatsD client also provides ability to send events, it would be nice to be able to just instantiate a single client instance, as part of the DogStatsD sync and reuse it externally for Event emitting purposes as well.

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