metrics.Gauge should get an Add method

[hardcoar]

type Gauge interface from pkg github.com/go-kit/kit/metrics has no field or method Add

hi guys i need help. i want to have reference on the current gauge using native prometheus (i don't want to implement it myself). since only method available on the interface in metrics package is Set i don't have any reference to the current gauge number.

is strange that pkg metrics/prometheus has Add method and pkg metrics don't?
is there an opinionated way to do this and am i missing something?
is it a generalization for other collectors aside from prometheus?
what's the correct way to Add in gauge (for prometheus)?

[peterbourgon]

It was my original opinions that Gauges should only support the Set method. But I've discovered that many important uses of Gauges are modeled by +N/-N semantics. For example, consider inflight requests. Right now, you'd have to do

func (s *Server) Handle() {
    s.Inflight.Set(atomic.AddInt64(&s.inflight, 1))
    defer s.Inflight.Set(atomic.AddInt64(&s.inflight, -1))
    // ...
}

versus the much nicer

func (s *Server) Handle() {
    s.Inflight.Add(1)
    defer s.Inflight.Add(-1)
    // ...
}

In the short term you can do as above. In the medium term I think I will need to re-add the Add method to the Gauge interface. Marking this as TODO.

[peterbourgon]

Closed in #428. Huge thanks to @dlmiddlecote! Amazing work.