nil pointer dereference for workersRegistered in Prometheus emitter

[cirocosta]

Hey,

In the last PR that got merged regarding updating the workersRegistered metric (see #3076), I missed providing the instantiation of the GaugeVec to the emitter struct.

Naturally, this leads Prometheus scraping to make the whole web process die with a SIGSEGV:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xc85707]

goroutine 45 [running]:
github.com/prometheus/client_golang/prometheus.(*GaugeVec).GetMetricWithLabelValues(...)
	/tmp/build/70f2e240/gopath/pkg/mod/github.com/prometheus/client_golang@v0.9.2/prometheus/gauge.go:183
github.com/prometheus/client_golang/prometheus.(*GaugeVec).WithLabelValues(0x0, 0xc0082f4dd0, 0x1, 0x1, 0xc0059674c8, 0x1c3da01)
	/tmp/build/70f2e240/gopath/pkg/mod/github.com/prometheus/client_golang@v0.9.2/prometheus/gauge.go:215 +0x27
github.com/concourse/concourse/atc/metric/emitter.(*PrometheusEmitter).workersRegisteredMetric(0xc00043fc20, 0x24ac120, 0xc003df83c0, 0x1c46bce, 0xc, 0x192bcc0, 0xc004121970, 0x1c3984f, 0x2, 0xc0027e2900, ...)
	/tmp/build/70f2e240/concourse/atc/metric/emitter/prometheus.go:473 +0x100
github.com/concourse/concourse/atc/metric/emitter.(*PrometheusEmitter).Emit(0xc00043fc20, 0x24ac120, 0xc003df83c0, 0x1c46bce, 0xc, 0x192bcc0, 0xc004121970, 0x1c3984f, 0x2, 0xc0027e2900, ...)
	/tmp/build/70f2e240/concourse/atc/metric/emitter/prometheus.go:337 +0x150
github.com/concourse/concourse/atc/metric.emitLoop()
	/tmp/build/70f2e240/concourse/atc/metric/emit.go:142 +0x133
created by github.com/concourse/concourse/atc/metric.Initialize
	/tmp/build/70f2e240/concourse/atc/metric/emit.go:102 +0x1f2

The fix is rather simple - give the instantiated object to the PrometheusEmitter instance and that's fixed.

This issue is meant more to document how little things like this, at the moment, are able to greatly affect the execution of web instances - call out to #2896