fix:fix data race bug when collecting metrics

[Sniper91]

reproduce procedure

1. run kafka exporter with race detector

go run -race ./  --kafka.server localhost:9092

2. request kafka metrics frequently

curl http://localhost:9308

output

==================
WARNING: DATA RACE
Read at 0x00c0001ab170 by goroutine 27:
  main.(*Exporter).collectChans()
      /Users/bytedance/opensource/kafka_exporter/kafka_exporter.go:355 +0x366
  main.(*Exporter).Collect.func1()
      /Users/bytedance/opensource/kafka_exporter/kafka_exporter.go:330 +0x5b

Previous write at 0x00c0001ab170 by goroutine 28:
  main.(*Exporter).collectChans.func1()
      /Users/bytedance/opensource/kafka_exporter/kafka_exporter.go:347 +0x267

Goroutine 27 (running) created at:
  main.(*Exporter).Collect()
      /Users/bytedance/opensource/kafka_exporter/kafka_exporter.go:330 +0x517
  github.com/prometheus/client_golang/prometheus.(*Registry).Gather.func1()
      /Users/bytedance/opensource/kafka_exporter/vendor/github.com/prometheus/client_golang/prometheus/registry.go:455 +0x245

Goroutine 28 (finished) created at:
  main.(*Exporter).collectChans()
      /Users/bytedance/opensource/kafka_exporter/kafka_exporter.go:345 +0x253
  main.(*Exporter).Collect.func1()
      /Users/bytedance/opensource/kafka_exporter/kafka_exporter.go:330 +0x5b
==================