panic: duplicate label names

Looks like the exporter crashes when a metric has duplicate label names.

time="2020-05-04T13:49:10Z" level=info msg="Starting stackdriver_exporter (version=0.7.0, branch=HEAD, revision=a339261e716271d77f6dc73d1998600d6d31089b)" source="stackdriver_exporter.go:136"
time="2020-05-04T13:49:10Z" level=info msg="Build context (go=go1.14.2, user=root@6bfda044714a, date=20200501-12:39:15)" source="stackdriver_exporter.go:137"
time="2020-05-04T13:49:10Z" level=info msg="Listening on :9255" source="stackdriver_exporter.go:163"
panic: duplicate label names

goroutine 162 [running]:
github.com/prometheus/client_golang/prometheus.MustNewConstHistogram(...)
	/app/vendor/github.com/prometheus/client_golang/prometheus/histogram.go:619
github.com/prometheus-community/stackdriver_exporter/collectors.(*TimeSeriesMetrics).newConstHistogram(0xc000377a18, 0xc0005fe280, 0x50, 0xc000a47600, 0xe, 0x10, 0xc0003e4870, 0xc000a51da0, 0xc000a47700, 0xe, ...)
	/app/collectors/monitoring_metrics.go:94 +0x19a
github.com/prometheus-community/stackdriver_exporter/collectors.(*TimeSeriesMetrics).completeHistogramMetrics(0xc000377a18)
	/app/collectors/monitoring_metrics.go:186 +0x1c7
github.com/prometheus-community/stackdriver_exporter/collectors.(*TimeSeriesMetrics).Complete(0xc000377a18)
	/app/collectors/monitoring_metrics.go:149 +0x39
github.com/prometheus-community/stackdriver_exporter/collectors.(*MonitoringCollector).reportTimeSeriesMetrics(0xc0000b88f0, 0xc000940000, 0xc000356b00, 0xc0001a4f00, 0xc000940000, 0x0)
	/app/collectors/monitoring_collector.go:370 +0x10a3
github.com/prometheus-community/stackdriver_exporter/collectors.(*MonitoringCollector).reportMonitoringMetrics.func1.1(0xc000412640, 0xc0000b88f0, 0xe1193bb, 0xed6421353, 0x0, 0xe1193bb, 0xed642147f, 0x0, 0xc0003645a0, 0xc000356b00, ...)
	/app/collectors/monitoring_collector.go:223 +0x5e7
created by github.com/prometheus-community/stackdriver_exporter/collectors.(*MonitoringCollector).reportMonitoringMetrics.func1
	/app/collectors/monitoring_collector.go:197 +0x3f3

It seems like something introduced in v0.7.0 as I don't see the same issue in v0.6.0

Start in collectors/monitoring_metrics.go at newConstHistogram and follow its callers through completeHistogramMetrics and Complete. Reproduce a metric with duplicate label names and trace how the histogram is constructed; done means the exporter no longer panics for that input, with the expected handling confirmed from the issue discussion.