Http client

[stuartnelson3]

@beorn7 adding this here for to start discussion, it has been heavily informed by the middleware approach in #285

Current API:

client := *http.DefaultClient
client.Timeout = 500 * time.Millisecond

inFlightGauge := prometheus.NewGauge(prometheus.GaugeOpts{
	Name: "in_flight",
	Help: "An in-flight gauge",
})

counter := prometheus.NewCounterVec(
	prometheus.CounterOpts{
		Name: "test_counter",
		Help: "A test counter.",
	},
	[]string{"code", "method"},
)

histVec := prometheus.NewHistogramVec(
	prometheus.HistogramOpts{
		Name:    "latency",
		Help:    "A histogram of latency.",
		Buckets: prometheus.DefBuckets,
	},
	[]string{"event"},
)

prometheus.MustRegister(counter, histVec, inFlightGauge)

promclient := promhttp.InFlightC(inFlightGauge,
	promhttp.CounterC(counter,
		promhttp.ClientTrace(histVec, &client),
	),
)

resp, err := promclient.Get("example.com")
// And so forth ...

[stuartnelson3]

Closed in favor of #295