Fix: panic when max-length metrics are excluded

[j-gourdon]

This PR fix an exporter crash when excluding max-length and max-length-bytes metrics. (limitsGaugeVec is an empty map in this case)

Step to reproduce :

• Version : v1.0.0-RC13
• Set EXCLUDE_METRICS=max-length-bytes,max-length
• Scrape data

Panic output

[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x7c19fd]

goroutine 25 [running]:
github.com/prometheus/client_golang/prometheus.(*GaugeVec).WithLabelValues(0x85cee0, {0xc001b111a0, 0x8cd992, 0xa})
        /home/runner/go/pkg/mod/github.com/prometheus/[email protected]/prometheus/gauge.go:215 +0x1d
main.exporterQueue.Collect({0xc0000cd800, 0xc0000cca50, 0xc0000cd950, 0xc0000a0318, 0xc0000a0320}, {0xa00eb8, 0xc0004904e0}, 0x4669d3)
        /home/runner/work/rabbitmq_exporter/rabbitmq_exporter/exporter_queue.go:219 +0xf69
main.(*exporter).collectWithDuration(0xc000452c80, {0x9fe870, 0xc00044fef0}, {0xc000024029, 0x5}, 0x0)
        /home/runner/work/rabbitmq_exporter/rabbitmq_exporter/exporter.go:142 +0x394
main.(*exporter).Collect(0xc000452c80, 0xc00049e0c0)
        /home/runner/work/rabbitmq_exporter/rabbitmq_exporter/exporter.go:107 +0x23b
github.com/prometheus/client_golang/prometheus.(*Registry).Gather.func1()
        /home/runner/go/pkg/mod/github.com/prometheus/[email protected]/prometheus/registry.go:446 +0x102
created by github.com/prometheus/client_golang/prometheus.(*Registry).Gather
        /home/runner/go/pkg/mod/github.com/prometheus/[email protected]/prometheus/registry.go:538 +0xb4d

[kbudde]

Thank you!