Support float64 for gauges

[vprithvi]

Requirement

We would like to emit gauge updates with floating numbers (e.g. probabilities)

Problem

The gauge interface in jaeger-lib only supports int64

jaeger-lib/metrics/gauge.go

Lines 18 to 21 in 78a0e7f

	type Gauge interface {
	// Update the gauge to the value passed in.
	Update(int64)
	}

Implementations like tally, prom, go-kit, influx and expvar support float64 gauges, but jaeger-lib casts the int64 to a float64 value. Is there a reason for this?

jaeger-lib/metrics/prometheus/factory.go

Lines 230 to 232 in 78a0e7f

	func (g *gauge) Update(v int64) {
	g.gauge.Set(float64(v))
	}

jaeger-lib/metrics/tally/metrics.go

Lines 49 to 51 in 78a0e7f

	func (g *Gauge) Update(value int64) {
	g.gauge.Update(float64(value))
	}

jaeger-lib/metrics/go-kit/metrics.go

Lines 49 to 51 in 78a0e7f

	func (g *Gauge) Update(value int64) {
	g.gauge.Set(float64(value))
	}

Proposal

Update Gauge to use float64 instead of int64 in jaeger-lib 3.0; alternatively jaeger-lib can declare new types GaugeV2, FactoryV2, etc and create an adaptor.

[yurishkuro]

+1, FactoryV2, FloatGauge

I suspect it's for historical reasons, the old statsd based m3 client didn't support floats.