Prometheus collector metricset fails when a label has ","

[vjsamuel]

Prometheus collector fails whenever there is a label that has a comma.

Ex:

apiserver_request_count{client="Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36",code="200",contentType="application/json",resource="applications",verb="GET"} 1

2017/04/07 02:57:49.521217 log.go:161: ERR recovered from panic while fetching 'prometheus/collector' for host 'api.system.svc.31.tess.io'. Recovering, but please report this: runtime error: index out of range.
2017/04/07 02:57:49.521297 log.go:162: ERR Stacktrace: goroutine 25 [running]:
runtime/debug.Stack(0x546c08a, 0x2b, 0xc420561728)
	/usr/local/Cellar/go/1.7.4_2/libexec/src/runtime/debug/stack.go:24 +0x79
github.com/elastic/beats/libbeat/logp.Recover(0xc42017a0c0, 0x5f)
	/go/src/github.com/elastic/beats/libbeat/logp/log.go:162 +0x138
panic(0x5242cc0, 0xc4200120f0)
	/usr/local/Cellar/go/1.7.4_2/libexec/src/runtime/panic.go:458 +0x243
github.com/elastic/beats/metricbeat/module/prometheus/collector.extractLabels(0xc4200d22e8, 0xce, 0x5435f0d)
	/go/src/github.com/elastic/beats/metricbeat/module/prometheus/collector/data.go:61 +0x205
github.com/elastic/beats/metricbeat/module/prometheus/collector.NewPromEvent(0xc4200d22d0, 0xe9, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
	/go/src/github.com/elastic/beats/metricbeat/module/prometheus/collector/data.go:44 +0x20f
github.com/elastic/beats/metricbeat/module/prometheus/collector.(*MetricSet).Fetch(0xc420514460, 0x37f1840f, 0x5f48ce0, 0x400e6df, 0x5f4ed00, 0x6156110)
	/go/src/github.com/elastic/beats/metricbeat/module/prometheus/collector/collector.go:72 +0x1f1
github.com/elastic/beats/metricbeat/mb/module.(*metricSetWrapper).multiEventFetch(0xc4202e0ea0, 0x6156110, 0xc420514460, 0xc4205a5c80, 0x1, 0xc42017a0c0, 0x5f, 0x200)
	/go/src/github.com/elastic/beats/metricbeat/mb/module/wrapper.go:273 +0x6c