Expose application metrics in a /metrics endpoint about fleet

[groob]

Now that we have a datastore abstraction, adding metrics on top would be a great place to start, and I'd like to tackle this sometime soon.

Here's what I'd like to do.

package datastore

// a private type that embeds the datastore interface
type instrumented struct {
    count metrics.Counter
    latency metrics.TimeHistogram
    Datastore
}

accept a datastore and return back the above one

func NewWithMetrics(ds Datastore) Datastore {
    count := metrics.NewCounter(...)
    latency := metrics. NewTimeHistogram(...)

    return instrumented{count, latency, ds}
}

layer methods on top of datastore

func (mw instrumented) NewUser(user *kolide.User) (user *kolide.User, err error) {
// from go-kit metrics example  
defer func(begin time.Time) {
        methodField := metrics.Field{Key: "method", Value: "newuser"}
        errorField := metrics.Field{Key: "error", Value: fmt.Sprintf("%v", err)}
        mw.requestCount.With(methodField).With(errorField).Add(1)
        mw.requestLatency.With(methodField).With(errorField).Observe(time.Since(begin))
  }(time.Now())
  user, err = mw.Datastore.NewUser(user)
}

[marpaia]

This should be easy to do now that we're using go-kit.

[groob]

Closing as I've begun initial work in #236
Will continue going down this path from now on.